What is the doctrine of the Trinity?

Question

What is the doctrine of the Trinity as defined by Christianity?

Obviously there are three entities involved: God the Father, Christ, and the Holy Spirit, but is there more to the definition?

Answer 1

As you already understand, the doctrine of the Trinity states that there is one God, who exists in three persons. The Father, the Son, and the Holy Spirit.

From http://carm.org/trinity

The Father is not the same person as the Son; the Son is not the same person as the Holy Spirit; and the Holy Spirit is not the same person as Father. They are not three gods and not three beings. They are three distinct persons; yet, they are all the one God. Each has a will, can speak, can love, etc., and these are demonstrations of personhood. They are in absolute perfect harmony consisting of one substance. They are coeternal, coequal, and copowerful. If any one of the three were removed, there would be no God.

...

There is, though, an apparent separation of some functions among the members of the Godhead. For example, the Father chooses who will be saved (Eph. 1:4); the Son redeems them (Eph. 1:7); and the Holy Spirit seals them, (Eph. 1:13).

I'd like to expand on that, but there's not much I could expand on that hasn't already been covered in depth on this site. And while it's really not a complex doctrine, all sorts of heretical teachings come about based on a misunderstanding of it, or a rejection of it. Rather than going into any more detail here, I'd just recommend browsing the various questions tagged "Trinity" here.

Answer 2

The following is an attempt to alleviate much of the confusion in the Christian, 21st century mind, regarding the Doctrine of the Trinity and the confused idea that "God is one Person and the Father, Son and Holy Spirit are each Persons, yet we do not have four persons but three and not three Gods, but one. The old 3 in 1 and 1 in 3 problem." Much of what follows comes from the Westminster Confession of Faith and the Westminster Shorter Catechism.

The Trinity

The Father, Son and Holy Spirit are 3 separate Persons (i.e. each have their own consciousness). They each partake of one essence or nature (that is what makes each Person to be God). The nature of God must never be confused with the 3 Persons, Who are each God. The nature of God is not a person, otherwise we have 4 separate persons. The Bible teaches one essence, or nature, and 3 Persons. (As an aside, when speaking with Muslims, one needs to be clear on this point). It is confusion to think that three are one and one is three in the same sense. To say that there is One Person Who is God and Three Persons Who are God, is nonsense. And the Bible never makes this mistake. Neither do the creeds (like the Nicene), Calvin, or the later Confessions such as the Westminster Confession (WCF). (As an example from the WCF, consider the following, (speaking about the two natures of Christ), where the phrase "and distinct natures, the Godhead and the manhood" obviously teaches that the Godhead refers to the nature of the Person, not to an actual Person, just as manhood does not refer to a person, but the nature of an individual)....

"The Son of God, the second person of the Trinity, being very and eternal God, of one substance and equal with the Father, did, when the fullness of time was come, take upon Him man's nature, with all the essential properties, and common infirmities thereof, yet without sin; being conceived by the power of the Holy Ghost, in the womb of the virgin Mary, of her substance. So that two whole, perfect, and distinct natures, the Godhead and the manhood, were inseparably joined together in one person, without conversion, composition, or confusion. Which person is very God, and very man, yet one Christ, the only Mediator between God and man"

Essence or nature can be applied to anything. It is akin to a definition. Unfortunately, it is almost impossible to define anything since we are so slow of understanding. For a simple example, Let's define a chair? Does it have 3 legs and a round top? What about 4 legs and a square top? What about steel tubes that curve around like a "S"...and so on. I'm sure you get the picture. It is very hard to fully define even something simple, like a chair!

So moving on from chairs: What is a rock? What is a tree? What is a human? Does a human need two arms? Does a human have to be >3 feet tall? Does a human have to have a certain IQ? In all these examples we are trying to understand what set of characteristics make up the nature of whatever we are trying to define. i.e. What is the nature or essence of a chair (or a rock or a tree or a human)? You will see that the essence of anything is extremely hard to define, even for a chair! Yet, the essence has to be complete in order for that entity to be really all that that entity is. If you take away part of the definition (remove part of the nature) then you have something else. e.g. take away the FULL definition of a chair and you no longer have a chair, you have something else whatever that may be, but you no longer have a chair. The same is true of addition to the nature.

God & nature (or essence)

We are shown clearly what one in essence and more than one person means from the very first chapter of the Scriptures. In Genesis 1 we read, "So God created MAN in His own image; in the image of God He created him; male and female He created them." (Emphasis added) We have MAN and we have two persons who are MAN, Adam and Eve. One MAN, two persons. It is the same with everything else, e.g. One TREE, individual gums, pines and willows. Yet we call them each a tree. One GOD, three Persons Who are GOD, the Father, the Son and the Holy Spirit.

Now apply this to God. What does it mean to be an entity called God (from the Bible)? Summarised, it means to be a Spirit, Who is infinite, eternal and unchangeable, in His being, wisdom, power, holiness, justice, goodness and truth, (obviously not a full definition). It is extremely hard to get a full definition of what it means to be God. But scouring the Bible leads us to at least this summary, as just given.

Now the three Persons, Father, Son and Spirit each have ALL the characteristics just mentioned (Spirit, infinite, eternal, unchangeable etc). And ONLY these 3 Persons have those characteristics. Nothing else at all (even in our wildest imaginations of sci-fi characters) has these characteristics, otherwise those things would also be God. This full set of characteristics comprise the essence of what it means to be God i.e. it describes the nature of a being we call God

The Athanasian creed though is even more circumspect here. And we need to be careful that we do not think that if Father, Son and Holy Spirit are each omnipotent, then we have three omnipotents. No. The nature or essence cannot be divided like that. The three Persons are one in their omnipotence. They are one in their wisdom. One in their eternity, One in their holiness etc.

(Think about another example. There is only one full definition of what it means to be a human. Yet we can see two people, say a brother and a sister. But we do not say that we have two humanities. We have two persons, but they are each fully one humanity or human).

If you were to ask me, "Who do you worship"? I would answer, "I worship three Persons, The Father, The Son and the Holy Spirit.' I do not worship a nature or essence or set of characteristics.

"But do you worship God as omnipotent, eternal etc?"

"Yes, I worship each of the three Persons as omnipotent, eternal etc."

"Doesn't that mean you worship 3 omnipotents, 3 eternals?"

"No. The three each partake fully of each characteristic e.g. Omnipotence, eternal. They do not each have a part of omnipotence. They are each fully and equally omnipotent. One omnipotence, each fully invested in each of the three persons."

I am one person. You are one person. But we each partake fully of what it means to be human.(Remember.....one set of human characteristics, but e.g. a brother and a sister).

For another example, there is an essence of a chair. But it does not mean that because we have two chairs or a dozen chairs then we have 2 or a dozen essences of chair. No, there is only one essence of what a chair is. And every chair fully partakes of that essence. The essence is not divided between the chairs. They each are a chair because they fully fit what it means to be a chair i.e. they each fully partake of the nature or essence of a chair.

Summary

So if an entity has a particular set of characteristics for God, then that entity must be God.... Just as if any object has ALL the characteristics of a chair, then we say that thing MUST be a chair. It cannot be a tree, since it does not have all the characteristics of what it means to be a tree.

That is also why I can say without reservation that you and I are not God. We simply do not have any of those characteristics (and we certainly don't have all of them). But the Father, Son and Spirit each have ALL of those characteristics summarised above. Thus the Father is God, the Son is God, the Spirit is God. They are equal in power and glory. They each have the same nature.

Part of the confusion about the Trinity today lies in two areas.

1. For many the doctrine is hard to understand and they say that indeed the doctrine of the Trinity cannot be understood at all. It is said to be "a mystery". But that is not what Athanasius, or the creeds, ever said. When the church fathers, or men such as Calvin, refer to the "mystery" of the Trinity they are referring to the deepness of the doctrine in terms of what comprises the essence of God. And to use modern language, the definition (or the nature) of what it means to be God is indeed mind-blowing [e.g. Just consider eternity (dwelling outside of time). A finite mind has trouble starting with that. But if the Father is the Creator (part of His nature), then He created time. But before time, He was, otherwise He could never have created it. But to be before time means He had no beginning (since you need the concept of time to posit a beginning), therefore God always was. i.e. He is eternal]. And ditto for the Son and Holy Spirit. This essence of what it means to be God is deep, very deep.

2. But this leads us to the second area of confusion today on this subject. It is hard when talking about God, not to use pronouns like He, Him, His etc. The problem arises when we translate (in our heads) from the pronouns to become a Person. The Bible never does that. In 99% of the time when the Bible refers to God, one can easily substitute the Father (Who is a Person) instead of the word "God". So far so good. But when the Bible is describing the nature (characteristics) of God, it is wrong to substitute "Person" for "God". That leads to the "One Person is three persons" confusion and then it is said to be a mystery that no one can understand!

The Bible is quite clear. There is one nature (set of characteristics) that makes up any entity Who is God. Just like there is one nature or essence (set of characteristics) that makes up a thing called a chair. There are many separate and quite distinct chairs, but they ALL have the same nature or essence (characteristics) of what it means to be A CHAIR (one chair). Thus the Father is a Person and He has ALL the nature (characteristics) of God. The Son is a Person and He too has ALL the characteristics of God and the Holy Spirit is a Person Who has all the characteristics of what it means to be God. Three Persons, one nature. Not three Persons, one Person.

If we do not separate out nature or essence from Persons, we will forever be in confusion. The church fathers had no such confusion. Jesus had no such confusion. We see that Jesus does not pray to God as an essence, but He specifically addresses the Father, or He sends the Spirit. To think of the essence as a person leads to confusion.

Thus the Bible teaches that the Lord is One (essence (NOT Person)) and that there are three Persons Who each have the full essence (full set of characteristics) that make up God. (I could have said ...full set of characteristics that make up an entity called God, but could not have said...full set of characteristics that make up a Person Who is God). The full set of characteristics (essence) are NOT a Person. Rather there are 3 Persons (Father, Son and Spirit) Who have all those characteristics of what it means to be God.

To show how the modern understanding of Deut 6:4 (the Lord our God, the Lord is one) is confusion, try reading this, substituting Tree for God: "Behold the Tree. The Tree is one. And the one tree is three, or there are three that are the one tree"....... Makes absolutely no sense, because I am keeping the same meaning (one tree) for the word "three".

If you read something like the Westminster Confession on this you will see what I am talking about, even from the chapter heading on this subject: "Of God, and of the Holy Trinity" >>>>>modern translation: "Of what it means to be God, and the Three Persons Who are each God". No confusion here.

When we think about God, we need to meditate on what the Bible teaches about the essence of (what it means) to be God (eternity, omnipotence etc) and then adore the Three Persons Who are each fully God (equal in one essence i.e. equal in power, glory, eternity, wisdom, power etc). We cannot separate out the essence, for then we do not have God (remember the chair). But we must separate out the Persons (Father, Son and Spirit) Who are each blessed forever and each fully God.

Addendum

John 10:30 "I and the Father are one"

Jesus cannot mean in this sentence that he and the Father are one person. That they are not one person is patently obvious, unless we think that when Christ prayed to the Father he was praying to Himself! ?? Rather, Jesus speaks to the Father and the Father speaks to Jesus. If they are not one person, then what does Jesus mean when he says that he and the Father are one?

The context is that he "is speaking of the impossibility of plucking any of the sheep, out of his own and his Father's hands; giving this as a reason for it, their unity of nature, and equality of power; so that it must be as impracticable to pluck them out of his hands, as out of his Father's, because he is equal with God the Father....". Thus the Father and the Son are God, in the same way that two brothers are Human.

Please consider the following commentators on this verse and how they explain the "Oneness of the Father and Jesus". You will note they refer to essence for oneness.

"Now, it is nowhere there said that believers and the Father are one, but such a statement is scrupulously avoided. Numerous attempts have been made to escape from the stupendous assumption of this unity of power and essence with the Father. The whole gist of the assertion reveals the most overwhelming self-consciousness. The Lord declares that he can bestow eternal life and blessedness upon those who stand in close living relation with himself, and between whom and himself there is mutual recognition and the interchanges of love and trust. He bases the claim on the fact that the Father's hands are behind his, and that the Father's eternal power and Godhead sustain his mediatorial functions and, more than all, that the Father's Personality and his own Personality are merged in one essence and entity. If he merely meant to imply moral and spiritual union with the Father, or completeness of revelation of the Divine mind, why should the utterance have provoked such fierce resentment"? See Here

"I and my Father are one. Not in person, for the Father must be a distinct person from the Son, and the Son a distinct person from the Father; and which is further manifest, from the use of the verb plural, "I and my Father", "we are one"; that is, in nature and essence, and perfections, particularly in power; since Christ is speaking of the impossibility of plucking any of the sheep, out of his own and his Father's hands; giving this as a reason for it, their unity of nature, and equality of power; so that it must be as impracticable to pluck them out of his hands, as out of his Father's, because he is equal with God the Father, and the one God with him". See here

"I and my Father are one—Our language admits not of the precision of the original in this great saying. "Are" is in the masculine gender—"we (two persons) are"; while "one" is neuter—"one thing." Perhaps "one interest" expresses, as nearly as may be, the purport of the saying. There seemed to be some contradiction between His saying they had been given by His Father into His own hands, out of which they could not be plucked, and then saying that none could pluck them out of His Father's hands, as if they had not been given out of them. "Neither have they," says He; "though He has given them to Me, they are as much in His own almighty hands as ever—they cannot be, and when given to Me they are not, given away from Himself; for He and I HAVE ALL IN COMMON." Thus it will be seen, that, though oneness of essence is not the precise thing here affirmed, that truth is the basis of what is affirmed, without which it would not be true. And Augustine was right in saying the "We are" condemns the Sabellians (who denied the distinction of Persons in the Godhead), while the "one" (as explained) condemns the Arians (who denied the unity of their essence)". See here

An excellent summary of the above with Scripture references is here.

For Further Reading: The best book I have read, by far, explaining clearly the Biblical Doctrine of the Trinity is by Dr Gordon H. Clark, The Trinity

Calvin in the 1500's taught what the Bible has to say on the subject (see Institutes, Book 1 Ch13). See also a Greek Orthodox explanation which is simple but Biblical.

The Trinity is often viewed as a difficult if not self-contradictory concept. Is the Trinity really incoherent? The following article responds with a simple but detailed answer to say "NO". https://trinityfoundation.org/journal.php?id=70

Answer 3

The doctrine of the Trinity is the logical construct whereby three of the most obvious but otherwise seemingly contradictory bits of Scripture are reconciled. Namely:

1. That the Creator of the Universe (God the Father), YHWH to the Jews was God

   Point one is not in dispute - the work of the Creator who proclaims himself to be God is God.

2. That Jesus, the Incarnate "son of God" was God

   Point two - that Jesus the Christ is God is the defining belief of a Christ-ian (regardless of what this site's definition may be!). That Jesus claimed to be God is pretty well established, both throughout the book of John, and in other questions on this site.

3. That there is only one God

   Point three - that there is only one God, is made throughout the Old Testament. You cannot be mono-theistic and believe in more than one God.

The basic definition of the Trinity was popularized and credalized in the Nicene Creed, in which monotheistic Christians declare belief in God the Father, God the Son, and God the Holy Spirit. Furthermore, the Nicene Creed says that God the Son is "of one Being with the Father," and that the Holy Spirit proceeds "from the Father through (or and) the Son." This is the most widely held formulation of the Trinity - and is accepted by all but the most fringe of Christians.

Reconciling that God is one, and yet exists as three distinct persons is the central "problem" of the Trinity, and most heresies about the nature of God come from over stressing either their oneness or their distinctness.

It is not tritheism (meaning that there are three Gods) nor is it one God who just appears in three different forms. It is not a matter of "1 + 1 + 1 = 1", but rather "1 x 1 x 1 = 1".

Some have said, that the Trinity can best be expressed as three individuals, so closely intertwined, as to be indistinguishable, and yet of such distinct personality as to form a community even within in itself. As in a marriage, where two become one, the Trinity reflects that closeness, and, since marriage is often likened to a sign of the Trinity, supercedes it. (Signs are less than the thing they point to.) Whatever the nature of the Trinity, it is a core doctrine of all Nicene Christians that rejects any idea that God is more than One and simultaneously less than Three. It is the "Three-In-One" Godhead.

It is always contentious. and other questions show why many analogies just don't work.

That this "mystery" has no earthly equivalent is not, however, an impossible defect - for it is simply understood that "God's ways are higher than our ways, and his thoughts higher than our thoughts." In other words, there is no law that says mortals need to be able to understand God's nature, just because there is no terrestrial equivalent.

Answer 4

On birth of Jesus Christ and His life, Death and Resurrection, it became essential to conceive the Doctrine of Trinity. Most obvious and direct evidence for Trinity originate from Bible depicting Jesus Christ and Holy Spirit with Nature of God. Therefore, if Father is God, Son is God and Holy Spirit is God than that’s it.

Father being God is acceptable to many which is as easy as accepting existence of God.

Jesus as God:
Instead of quoting all those verses from NT, where Jesus is depicted as same as God, here are few more events that leads us to know about this claim.

1. To quote Church historian Jaroslav Pelikan, he has pointed out that the oldest Christian sermon, the oldest account of Christian martyr, the oldest pagan report of the church, the oldest liturgical prayer, 1 Corinthians 16:22, all refer to Jesus as Lord and God.

Pelikan said, "Clearly, it was the message of what the church believed and taught that 'God' was an appropriate name for Jesus Christ." Do you see any possible way this could have developed, especially so soon, if Jesus had never made transcendent and messianic claims about himself? Not unless you're prepared to argue that the disciples completely forgot what the historical Jesus was like and that they had nothing to do with the traditions that start showing up twenty years after his death.”

.2. He was crucified precisely for this charge when He declared that He is the Son of God thus making Himself equal with God.

.3. Think of the story of Jesus walking in the water, found in Matthew 14:22-33 and Mark 6:45-52. Most English translations hide the Greek by quoting Jesus as saying, "Fear not, it is I." Actually, the Greek literally says, "Fear not, I am." Those last two words are identical to what Jesus said in John 8:58, when he took upon himself the divine name "I am," which is the way God revealed himself to Moses in the burning bush in Exodus 3:14. So Jesus is revealing himself as one who has the same divine power over nature as Yahweh, the God of the Old Testament.

.4. One of the most striking is his forgiving of sin. If you do something against me, I have the right to forgive you. However, if you do something against me and somebody else comes along and says, "I forgive you," what kind of cheek is that? The only person who can say that sort of thing is God himself, because sin, even if it is against other people, is first and foremost a defiance of God and his laws. So along comes Jesus and he says to sinners, "I forgive you." The Jews immediately recognize the blasphemy of this. They react by saying, "Who can forgive sins but God alone?" Not only did Jesus forgive sin, but he asserted that he himself was without sin. And certainly sinless-ness is an attribute of deity.

.5. The way God was going to save the world was by his Son dying. God, in his divine nature does not die. So He had to come as a human being to accomplish that task. And Jesus believed he was the one to do it. Jesus said in Mark 10:45, "I did not come to be served but to serve and give my life as a ransom in place of many." This is either the highest megalomania or it's the example of somebody who really believes, as he said, "I and the Father are one." In other words, "I have authority to speak for the Father; I have power to act for the Father; If you reject me, you've rejected the Father."

Holy spirit as God:
There are these verses and many more that depict Holy Spirit as the Person of Trinity.

1 Corinthians 2:11; 1 Corinthians 6:11; 1 Corinthians 6:19; 1 Corinthians 12:13; 2 Corinthians 3:5; 2 Corinthians 3:17-18; 2 Corinthians 13:13/14;Galatians 6:8; Ephesians 2:17-18; 1 Thessalonians 1:2-5; Hebrew 3:7-11; 2 Peter 1:21 etc.

1. 2 Cor 13:14 and Mt 28:19 both prove the personality of the Holy Spirit. In both passages, there is a "person, person, person" sequence. (Father, Son, Spirit).

2. 2 Cor 13:14 concludes with these important words: "be with you all". Three things are to be with you: "May Christ's grace be with you. May God's love be with you. May the Spirit's fellowship/sharing/companionship be with you." This exactly echoes the promise of the "comforter" in John 14:16-17, "And I [Jesus] will ask the Father, and He will give you another [Gr. allos: of the same kind as Jesus] Helper, that He [the Holy Spirit] may be with you forever; that is the Spirit of truth, whom the world cannot receive, because it does not behold Him or know Him, but you know Him because He abides with you, and will be in you." So Paul's desire in 2 Cor 13:14 for Christians to have the "fellowship of the Holy Spirit" is EXACTLY what Jesus promised in John 14:17 of the Spirit: "He abides with you, and will be in you".

3. The verse 2 Cor 13:14 says that the "person of Jesus Christ" is the source and supplier of grace, the "person of God" is the source and supplier of Love and the "person of the Holy Spirit" is the source and supplier of fellowship.

Conclusion:
Trinity is depicted in amorphous manner in OT and explicitly in NT. The reason for it being explicit in NT is when Jesus came into this world, the true nature of One Almighty God was also revealed through His message. God the father was revealed in OT. Second person was revealed by arrival of Jesus and the third person was revealed after Jesus was glorified. That is why we find most of the references to God the father in OT, God the Son in Gospels and majority of the references in explaining God the Holy spirit, after Jesus was glorified.

Answer 5

The doctrine of the Trinity as defined by Christianity is three persons in one Godhead.

God is a Trinity. The Bible says so.

The Trinity is revealed in the New Testament not in the Old Testament.

Deuteronomy 29:29 (NIV)

The secret things belong to the LORD our God, but the things revealed belong to us and to our children forever..

Proverbs 25:2 (NIV)

It is the glory of God to conceal a matter...

The Lord Jesus did clearly reveal that there exists three persons who've the same nature. He revealed this by making known the one name of the three persons.Their one name is expressive of their one nature ( cf: Exodus 3:14).

Matthew 28:19 (ASV)

Go ye therefore, and make disciples of all the nations, baptizing them into the name of the Father and of the Son and of the Holy Spirit:

God did create both male and female. This two people were given one name and this one name expresses their one nature.

Genesis 5:2 (ASV)

male and female created he them, and blessed them, and called their name Adam, in the day when they were created.

We should call the male and female who have the same name collectively as 'two humans' because of their human nature (human genome) which consists of parts but we should not call the three persons who have the same name as 'three gods' because their one divine nature is spiritual and hence, indivisible.

John 4:24 (NIV)

24 God is spirit, and his worshipers must worship in the Spirit and in truth.”

Answer 6

I don't know if I can add anything useful, but there are some different things I can say.

One might wonder why it makes sense for God to be Trinity. The only discussion of this question that I have ever read is found in Kallistos Ware's The Orthodox Way. He writes, in part:

“I believe in one God”: so we affirm at the beginning of the Creed. But then at once we go on to say much more than this. I believe, we continue, in one God who is at the same time three, Father, Son, and Holy Spirit. There is in God genuine diversity as well as true unity. The Christian God is not just a unit but a union, not just unity but community. There is in God something analogous to “society”. He is not a single person, loving himself alone, not a self-contained monad or “The One”. He is triunity: three equal persons, each one dwelling in the other two by virtue of an unceasing movement of mutual love. Amo ergo sum, “I love, therefore I am”

Why, then, believe in God as Trinity? [The] two most helpful ways of entry into the divine mystery are to affirm that God is personal and that God is love. Now both these notions imply sharing and reciprocity. First, a “person” is not at all the same as an “individual”. Isolated, self-dependent, none of us is an authentic person but merely an individual, a bare unit as recorded in the census. Egocentricity is the death of true personhood. Each becomes a real person only through entering into relation with other persons, through living for them and in them. There can be no man, so it has been rightly said, until there are at least two men in communication. The same is true, secondly, of love. Love cannot exist in isolation, but presupposes the other. Self-love is the negation of love. As Charles Williams shows to such devastating effect in his novel Descent into Hell, self-love is hell; for, carried to its ultimate conclusion, self-love signifies the end of all joy and all meaning. Hell is not other people; hell is myself, cut off from others in self-centeredness.

The doctrine of the Trinity was first articulated for the entire Church at the First Ecumenical Council in Nicaea in 325. The Nicene Creed affirmed what all Christians believe about the Trinity and its individual Persons:

We believe in one God, the Father Almighty, maker of all things visible and invisible; and in one Lord Jesus Christ, the Son of God, the only-begotten of his Father, of the substance of the Father, God of God, Light of Light, very God of very God, begotten (γεννηθέντα), not made, being of one substance (ὁμοούσιον, consubstantialem) with the Father. By whom all things were made, both which be in heaven and in earth. Who for us men and for our salvation came down [from heaven] and was incarnate and was made man. He suffered and the third day he rose again, and ascended into heaven. And he shall come again to judge both the quick and the dead. And [we believe] in the Holy Ghost. And whosoever shall say that there was a time when the Son of God was not (ἤν ποτε ὅτε οὐκ ἠν), or that before he was begotten he was not, or that he was made of things that were not, or that he is of a different substance or essence [from the Father] or that he is a creature, or subject to change or conversion—all that so say, the Catholic and Apostolic Church anathematizes them.

The Creed was amended at the Second Ecumenical Council in Constantinople in 381, to affirm that the Lord's "Kingdom shall have no end" (a refutation of the heretical teaching of Marcellus of Ancyra) and affirmed the Holy Spirit:

And [we believe] in the Holy Ghost, the Lord and Giver-of-Life, who proceedeth from the Father, who with the Father and the Son together is worshipped and glorified, who spake by the prophets.

The earliest dogmatic theology that was undertaken within the united Church (i.e. prior to the East-West schism of 1054) that I am aware of is John of Damascus' An Exact Exposition of the Orthodox Faith, a work written in Greek in the 8th century. In it he includes a large section entitled, "Concerning the Holy Trinity", wherein he explains aspects of the relationship between the three Divine Persons:

[All] that the Son and the Spirit have is from the Father, even their very being: and unless the Father is, neither the Son nor the Spirit is. And unless the Father possesses a certain attribute, neither the Son nor the Spirit possesses it: and through the Father, that is, because of the Father’s existence, the Son and the Spirit exist, and through the Father, that is, because of the Father having the qualities, the Son and the Spirit have all their qualities, those of being unbegotten, and of birth and of procession being excepted. For in these hypostatic or personal properties alone do the three holy subsistences differ from each other, being indivisibly divided not by essence but by the distinguishing mark of their proper and peculiar subsistence.

Further we say that each of the three has a perfect subsistence, that we may understand not one compound perfect nature made up of three imperfect elements, but one simple essence, surpassing and preceding perfection, existing in three perfect subsistences. For all that is composed of imperfect elements must necessarily be compound. But from perfect subsistences no compound can arise. Wherefore we do not speak of the form as from subsistences, but as in subsistences4. But we speak of those things as imperfect which do not preserve the form of that which is completed out of them. For stone and wood and iron are each perfect in its own nature, but with reference to the building that is completed out of them each is imperfect: for none of them is in itself a house.

The subsistences then we say are perfect, that we may not conceive of the divine nature as compound. For compoundness is the beginning of separation. And again we speak of the three subsistences as being in each other, that we may not introduce a crowd and multitude of Gods. Owing to the three subsistences, there is no compoundness or confusion: while, owing to their having the same essence and dwelling in one another, and being the same in will, and energy, and power, and authority, and movement, so to speak, we recognise the indivisibility and the unity of God. For verily there is one God, and His word and Spirit.