Many historians and religious scholars ... attest to the influence of Greek or Platonic philosophy in the development and acceptance of the Trinity doctrine in the fourth century. ... the famous Greek philosopher Plato ... believed in a divine triad of "God, the ideas, [and] the World-Spirit,". (Source)
The easiest way to verify the authenticity of this claim would be to check if any of the early church fathers believed in trinity. (Please note by early I mean the <= 300 AD)
The Apostles Creed, referenced as early as 390 A.D., quite clearly articulates an understanding of our faith revolving around the three persons we identify as the members of the Trinity as core to the faith. It makes up almost half of the total text.
I believe in God, the Father Almighty,
the Maker of heaven and earth,
and in Jesus Christ, His only Son, our Lord:Who was conceived by the Holy Ghost,
born of the virgin Mary,
suffered under Pontius Pilate,
was crucified, dead, and buried;
It is clearly understood both today and in its time that this creed was affirming a belief in One God and at the same time identifying how he is revealed to us. While it doesn't use the term Trinity nor define the mixed natures as clearly as later creeds and confessions, it is unlikely that such a creed would have seen the near universal acceptance as articulating the core of Christian belief if it went against the teachings of the church fathers of the previous three centuries. The lack of controversy on this topic is remarkable. We do know people who taught otherwise, such as the Gnostics and Arians, but part of the purpose of this Creed was to emphatically reject them, a standard that has continued throughout mainstream Christian history.
Before moving on, it would be remiss not to point out that the most emphatic and authoritative writings on this matter in the first couple centuries are in fact the Scriptures themselves. Looking in material written by "early fathers" for clues about what they did or did not believe without first and foremost considering the teachings found in the pages of the Bible their actions as church fathers helped to canonize would be a disservice to them.
That being said we do have clear evidence that the early Fathers did believe in the Trinity. Few of them used that word, but the words they did use fit the formula. For example Polycarp lived from 70 A.D to somewhere in the mid second century. Here's how he prayed:
"O Lord God almighty... I bless you and glorify you through the eternal and heavenly high priest Jesus Christ, your beloved Son, through whom be glory to you, with Him and the Holy Spirit, both now and forever."
While that statement was simply an affirmation of the three persons without directly trying to define their relation to each other, other early Fathers such as Theophilus from the late second century did remark on how they relate:
It is the attribute of God, of the most high and almighty and of the living God, not only to be everywhere, but also to see and hear all; for he can in no way be contained in a place.... The three days before the luminaries were created are types of the Trinity, God, his Word, and his Wisdom.
Likewise Justin (another Martyr and church leader from the second century) had this to say about baptism:
"For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water."
Clearly the three persons were at the center of the faith people were being inaugurated into. In another place, he comments specifically on the relation of these three persons to each other:
We will prove that we worship him reasonably; for we have learned that he is the Son of the true God himself, that he holds a second place, and the Spirit of prophecy a third. For this they accuse us of madness, saying that we attribute to a crucified man a place second to the unchangeable and eternal God, the Creator of all things; but they are ignorant of the mystery which lies therein.
By the first decade of the third century, we have quite a collection of declarations about the Trinity. Here is Tertullian:
And at the same time the mystery of the oikonomia is safeguarded, for the unity is distributed in a Trinity. Placed in order, the three are the Father, Son, and Spirit. They are three, however, not in condition, but in degree; not in being, but in form; not in power, but in kind; of one being, however, and one condition and one power, because he is one God of whom degrees and forms and kinds are taken into account in the name of the Father, and of the Son, and of the Holy Spirit.
The list goes on. I can't quote them all here but Ignatius the Bishop of Antioch, Irenaeus, a disciple of Polycarp and later Bishop of Lyons, Didache, Hippolytus, Novatian, Origen, and other Fathers through the second and third centuries can all be quoted with emphatic articulations of the Trinity.
The exact formulation and best way to articulate this mystery has been widely debated in every age and language since. The above Fathers certainly had varying ideas about just what it meant to have a single God with three persons. We still struggle to wrap our heads around this today. And that's ok as long as we do not err on the side of either denying either the unity or diversity of his nature. We must not make out the persons of the Trinity to be separate beings nor deny that the one being is revealed to us in thee persons.
To quote the wikipedia article on the Trinity:
The first of the early church fathers recorded as using the word Trinity was Theophilus of Antioch writing in the late second century. He defines the Trinity as God, His Word (Logos) and His Wisdom (Sophia) in the context of a discussion of the first three days of creation. The first defence of the doctrine of the Trinity was in the early third century by the early church father Tertullian. He explicitly defined the Trinity as Father, Son, and Holy Spirit and defended the Trinitarian theology against the "Praxean" heresy.
Theophilus of Antioch died around 185 AD. He did not use the word "trinity" to directly describe the triune nature of God as we understand and believe today, but it was in reference to God "God, his Word (Logos) and his Wisdom (Sophia)."
It seems from the history with which I am familiar, that there were other pressing matters before the church in the first couple centuries, and a formal adoption of the term "trinity" and 'officialization' of the doctrine as a stance which the church took did not happen until the Council of Nicaea in 325 AD, and it was taken to directly address the heresy of Arianism, which loosely states that Jesus is not equal to God, rather was created by God for the purpose of redemption.
Now - as to whether others taught it in recorded writings before the early 4th century, we do not know today. Trinitarians today will argue that because the doctrine is well-supported by both Christ and the Apostles that it must have been widely believed in the church, and only needed to be "addressed" when it came under attack (personally, I am of this belief), as was the case with some Apostolic writings wherein specific heresies are addressed (eg the Judaizers in Galatians).
You can find evidence of trinitarian beliefs scattered throughout the Church father's writings and prescriptions, particularly those regarding baptism.
But, I'd posit you need to look no further than the gospels to show a belief in the Trinity. Specifically and most notably, refer to Matthew 28:19.
"make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit"
Here we have Christ being quoted, not as using a term like "trinity", but speaking effectively of a trinity in a manner equating its members, at least one of which is commonly understood to be God by the readers. And while this is given as a quote from Christ, we must recognize that the gospels were not written by Christ; they were compiled by the first believers, the Church Fathers.
A thorough review of the gospels would reveal there are no vain details included. And, they're written by the early believers (Church Fathers). Hence, we can reasonably assume that distinct trinitarian language in the gospels is a clear indication of trinitarian beliefs in the early Church.
From what I've read (mainly in the Wikipedia article on 'Arianism'), There was a controversy between two interpretations of Jesus' divinity (Homoousianism and Arianism) based upon the theological orthodoxy of the time, one Trinitarian and the other also a derivative of Trinitarian orthodoxy. Arianism (Koinē Greek: Ἀρειανισμός, Areianismós) is a Christological doctrine first attributed to Arius (c. AD 256–336), a Christian presbyter from Alexandria, Egypt. So it goes back before the mentioned dates.
Arianism holds that the Son is distinct from the Father and therefore subordinate to Him. (Similar to the Gnostics I guess in that respect.) Homoousianism was formally affirmed by the first two ecumenical councils; since then, Arianism has always been condemned as "the heresy or sect of Arius". So now currently all mainstream branches of Christianity now consider Arianism to be heterodox and heretical.
Council of Nicaea of 325, convened by Emperor Constantine to ensure church unity, declared Arianism to be a heresy. According to Everett Ferguson,
"The great majority of Christians had no clear views about the nature of the Trinity and they did not understand what was at stake in the issues that surrounded it."
Constantine ordered the death penalty for those who refused to surrender the Arian writings:
In addition, if any writing composed by Arius should be found, it should be handed over to the flames, so that not only will the wickedness of his teaching be obliterated, but nothing will be left even to remind anyone of him. And I hereby make a public order, that if someone should be discovered to have hidden a writing composed by Arius, and not to have immediately brought it forward and destroyed it by fire, his penalty shall be death. As soon as he is discovered in this offence, he shall be submitted for capital punishment. ...
( Edict by Emperor Constantine against the Arians )
Constantine was the original culture-canceler, it seems! :P The impact of the Roman Empire taking up Christianity as its state religion cannot be underestimated. It dramatically changed Roman life, and also dramatically changed Christianity. For instance by 401AD, the punishment for heresy (aka not being Trinitarian) was decided to be death by fire (burnt alive at the stake) in the Church of England. To me that is a FAAAR cry from "love your enemies" and "judge not"- my guess is that was Roman influence. I could be wrong though. Maybe people just expressed their love differently back then. =/
Sources:
The victors write the history, and the losers become labeled heretics. The "Trinity" was always a controversial subject and the idea that Jesus and God were one and the same was also always a controversial subject. The Trinity didn't become orthodoxy until the Pagan Emperor (NEVER a Christian) Constantine shoved it down Christianity's throat.
Constantine worshiped Sol Invictus and at that time it was a tenant of that pagan religion that ALL GODS were an expression of Sol Invictus. Constantine basically surrendered to the fact the Christianity swept over the empire and was the dominant religion. He needed to make peace with the Christians and bring them into the fold if he was to preserve the Empire. There was a titanic problem, however; Christianity back then was almost as diverse as it is today (there being - no lie- about 43,500 different Christian denominations worldwide today). He needed Christianity united and so HE called the first council at Nicea in 325 AD and demanded that the Christian bishops get their act together and form a single dogma/statement of faith and get their religion under one umbrella.
Well, they couldn't. The entire block of Bishops from the east (Greece, Armenia, modern day Turkey, etc.) held to a tradition where God was God and Jesus was NOT God, but his son, separate and distinct from him. Noetus of Smyrna (circa 190 AD) got booted out of the church for declaring God and Jesus as one and the same. Noetus came and settled in Rome where he picked up followers to follow his philosophy.
The Trinity also falls into this discussion where it was fiercely argued against. Prior to 325 AD it was one church arguing against another and then Constantine ended the argument at Nicea where he got tired of the arguing and stepped in to declare that God the Father, the Son and the Holy Spirit were all expressions of one god as he believed and learned from his religion of Sol Invictus. Arius, the famous church historian Eusebius and other bishops who did not believe in the Trinity or that God and Jesus were one and the same were kicked out of the Council for disagreeing with the Emperor and the Bishops who did believe in the Trinity. With these expelled bishops left the entire eastern contingent of bishops.
Years later Eubebius was able to get Constantine to reverse his edict but the seeds were already set. By 390 AD the Roman Emperor Theodosius was thoroughly Christian (unlike Constantine) and his brand of Christianity embraced the Trinity "denomination", and his radical belief in it caused him to declare all other brands of Christianity to be heresies punishable by death if anyone was caught practicing, promulgating any of these other denominations or caught using any other scriptures other than the canon (which was imposed on Christianity by Constantine and Theodosius).
It started another round of persecutions of Christians who didn't adhere to Theodosius' orthodoxy. This is why the Nag Hammadi scrolls were buried by those who regarded them as inspired because if caught with them they would have been put to death. The Trinitarian Orthodoxy has nothing to do with the truth, but more to do with the victor writing the history. Luckily enough fragments survive of the oppositions' views. In this scholar's opinion, the first Apostasy was the idea God and Jesus were one and the same and the Trinity falls right in there too with God now being three expressions.
Eusebius, quoted Matthew 28:19 19 times. He quoted it 18 times without "the Father, the Son, and the Holy Spirit". The 1 time that he quoted it with "The Father, the Son , and the Holy Spirit", he wrote that he got it from Theodotus, a Gnostic.
No complete NT manuscripts, in Greek, are available before the 4th Century. They were burned and I'm 100% certain that it was done by Gnostics with trinitarian beliefs to cover up the fact that they added words to the NT.
"In the only codices which would be even likely to preserve an older reading, namely the Sinaitic Syriac and the oldest Latin Manuscript, the pages are gone which contained the end of Matthew (F.C. Conybeare)." Concerning Matthew 28:19, "it must be remembered that the African old Latin and of the old Syriac versions are 'defective at this point' (Encyclopedia of Religion and Ethics). I am 100% certain that it was done by trinitarians to cover up the fact that words were added to the NT.
The Apostles Creed was put into verbal form by the Apostles in the first century and it was put into written form as early as the second century. According to the text, the Holy Spirit conceived Jesus in Mary the Blessed, who was a virgin. The text is in agreement with what Apostle Matthew wrote in his account of the Gospel (Matthew 1:18). The fact the the Holy Spirit conceived Jesus means that the Holy Spirit is the Father. According to Apostle John, God is the Father of Jesus (John 3:16). According to Luke (the Disciple of Apostle Paul), the Holy Spirit is God (Acts 5:1-5). It is clear that the Father and the Holy Spirit is the same person and NOT two persons. The Creeds that came about after the 3rd Century are in contradiction with the Apostles Creed. Furthermore, the New Testament clearly shows distinction between the Father and the Son, which means that they are two separate beings. Logically, two separate beings can NOT be the same being.
The Greek word "Theos" has 3 meanings: God, king, and divine. Thomas did NOT state to Jesus "my Lord and my God" in John's Gospel account; he stated to Jesus "my Lord and my King" in John's Gospel account. Jesus is the Lord of lords and the King of kings! God the Father did NOT refer to Jesus as God in the Letter to the Hebrews; God the Father refereed to Jesus as King in the Letter to the Hebrews. Apostle John did NOT write "and the Word was God"; he wrote "and the Word was divine". God the Father and the Son of God are both divine. God the Father is a Deity but the Son of God is NOT a Deity. Adam and Eve are both human. Adam is a Man but Eve is NOT a man. In Genesis, it states that man and woman were made in the image and likeness of God and Memra (i.e. Logos). In the beginning was the rib and the rib was with Adam and the rib was human. In the beginning was the Logos and the Logos was with God and the Logos was divine. Adam and Eve are one. God the Father and the Son of God are one. Adam and Eve are equal but Eve is NOT the same being as Adam. God the Father and the Son of God are equal but the Son of God is NOT the same being as God. Apostle Paul wrote that though Jesus was divine (i.e. in the form of God), he did NOT consider equality something to be used to his advantage on earth and emptied Himself of His divinity. Apostle John wrote that the Logos entered the world as flesh through Mary the Blessed, who was a virgin. Jesus was 100% human on earth. Jesus was 0% divine on earth. The Hebrew word "Elohim" has 2 meanings: God and king. Isaiah wrote that a "mighty King" will be born and NOT a "mighty God" will be born. God has no beginning nor end! Jesus was created before time began!
Actually, it would be more appropriate to notice that many of the early church fathers were either Platonist or Neoplatonist, St. Augustine being the most Platonist of them all. In fact, his City of God is nothing but an allusion to and offshoot of Plato's Republic. And as far as belief of the trinity, these early church fathers actually derived the concept from Plato's works in general (note the many comparisons of Jesus to Socrates) but specifically, from such passages as this one from the opening of Plato's dialogue, Timaeus:
One, two, three, but where is the fourth, my dear Timaeus?
But one must needs understand what is implied by the "trinity." And it's not what the popular Christian idea tells us it is. God is One, and not three. Jesus is not God, the Father and Creator, but only the Son. There are so many passages in the Bible that logically make this point that it's goes beyond a shadow of doubt. The early church fathers knew this, even if some of their writings do not plainly point this out. The real meaning of the "trinity" is explained in Plato's works very clearly; it is referring to the "trice" removed of semblances (the physical, and sensible world) from the one reality, the ultimate Good (God). To try and explain in details and to the necessary length is beyond the scope of this answer. One needs to intently study the Bible, Plato, and many other works from many authors that stem from both authorities. But in a nutshell, there is definitely a connection between the Bible and Plato's works.
