For more than 3,000 years, Jews have repeated Deuteronomy 6:4. “Hear, O Israel: The Lord our God is one Lord.” This sacred passage has been held in high esteem and memorized by Jews for centuries.
Jesus also taught about “the only true God”
John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
The Oneness doctrine, however, overlooks the fact that the Son came to earth to reveal the true character of God the Father to a world groping in spiritual blindness.
Trinity is woven in scriptures:
On birth of Jesus Christ and His life, Death and Resurrection, it became increasingly self evident from the Bible itself the existence of Trinity or Triune God. Most obvious and direct evidence for Trinity emanated from Bible, depicting Jesus Christ and Holy Spirit with Nature of God. Therefore, to quote from the Athanasian Creed:
So the Father is God, the Son is God, the Holy Spirit is God; And yet there are not three Gods, but one God.”
If Father is God, Son is God and Holy Spirit is God than that’s it. There are numerous verses in Bible to vie for the Divinity of these three Persons in one God and that is how this answer has become such a long post.
Bible gives us irrefutable references about these claims. It never became that clear until the Son was revealed in NT. Second person was revealed by arrival of Jesus and the third person was revealed after Jesus was glorified. That’s how 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.
Jesus is same as God:
When Jesus prayed to His Father in Gethsemane, He said:
John 17:5-6 “And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was. I have manifested thy name unto the men which thou gavest me out of the world”
Jesus is the only one who could reveal the Father, because He is the express image of the Father (Luke 10:22; Thus when the disciples asked Christ what the Father was like, He could say:
John 14:9 Jesus replied, “Have I been with you for so long, and you have not known me, Philip? The person who has seen me has seen the Father! How can you say, ‘Show us the Father’?
Jesus so mirrored the character of the Father that He perfectly reflected Him, hence the title “The Everlasting Father”(Isaiah 9:6). I Another reason Jesus is called the Everlasting Father is because this world and everything in it was created through Christ. So in a very real sense, Jesus is our father:
Hebrews 1:2 in these last days he has spoken to us in a son, whom he appointed heir of all things, and through whom he created the world.
In Isaiah 9:6 Jesus is called the Father and here Old Testament refers Jesus to be same as Father.
Isaiah 9:6 For a child has been born to us, a son has been given to us. He shoulders responsibility and is called: Extraordinary Strategist, Mighty God, Everlasting Father, Prince of Peace.
Jesus also calls Himself the Son of man, our brother, our shepherd, our friend, and our priest. Jesus frequently said many things that left a clear impression about His "human" nature. Nonetheless, at many other places in Bible, Jesus makes us abundantly clear about His "Divine" nature as well. This was however an indispensable necessity for the promised Messiah-to be both human and Divine- as otherwise His death on cross would have no power to provide salvation for humankind.
Philippians 2:5-8 “Let this mind be in you, which was also in Christ Jesus: Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross”
As we compare Scripture definitions for God with the Bible record of Jesus, we see the characteristics of Jehovah are also ascribed to Jesus.
He is self-existent (John 1:1–4; 14:6); only God is self-existent (Psalm 90:2).
Jesus defines Himself as eternal. “I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty” (Revelation 1:8).
He is, and has, eternal life (1 John 5:11, 12, 20).
He is all-powerful (Revelation 1:8).
He created all things (John 1:3). “In the beginning God created the heaven and the earth” (Genesis 1:1). “For by Him all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him” (Colossians 1:16 NKJV).
The Father even calls Jesus God. “But unto the Son he saith, Thy throne, O God, is for ever and ever: a scepter of righteousness is the scepter of thy kingdom”
Hebrews 1:8-9. but of the Son he says, “Your throne, O God, is forever and ever, and a righteous scepter is the scepter of your kingdom. 1:9 You have loved righteousness and hated lawlessness. So God, your God, has anointed you over your companions with the oil of rejoicing.”
Jesus is able to forgive sin (Luke 5:20, 21); The Bible says only God can forgive sin (Isaiah 43:25).
Jesus accepted worship that according to the Ten Commandments is reserved only for the Almighty.
Matthew 14:33 Then they that were in the ship came and worshipped him, saying, Of a truth thou art the Son of God.
Matthew 28:9“And as they went to tell his disciples, behold, Jesus met them, saying, ‘All hail.’ And they came and held him by the feet, and worshipped him”
John 20:26–29 Upon seeing the risen Savior, the converted skeptic, Thomas, confessed, “My Lord and my God!”
Even the angels worship Jesus. “And again, when he bringeth in the first begotten into the world, he saith, And let all the angels of God worship him” (Hebrews 1:6).
The Scriptures also teach that only God knows the thoughts of a man’s heart (1 Kings 8:39). Yet Jesus consistently knew what people were thinking, “for he knew what was in man” (John 2:25). “Nathanael said to Him, ‘How do You know me?’ Jesus answered and said to him, ‘Before Philip called you, when you were under the fig tree, I saw you’” (John 1:48 NKJV).
Through the Spirit, Jesus is omnipresent. “Lo, I am with you always, even to the end of the age” (Matthew 28:20 NKJV). “For I am with you, and no one will attack you to hurt you; for I have many people in this city” (Acts 18:10 NKJV).
He has power to give life, and even resurrected Himself. “No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again” (John 10:18). “I am the resurrection, and the life: he that believeth in me, though he were dead, yet shall he live” (John 11:25).
John 10:30, 33“I and my Father are one,” Jewish leaders were outraged and sought to execute Him. They understood unequivocally that Jesus was claiming to be God Himself. “The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God”.
John 8:58 NKJV Jesus said to them, “‘Most assuredly, I say to you, before Abraham was, I AM.’ Then they took up stones to throw at Him; but Jesus hid Himself and went out of the temple, going through the midst of them, and so passed by”.
John 5:17, 18 “‘My Father worketh hitherto, and I work.’ Therefore the Jews sought the more to kill him, ... but said also that God was his Father, making Himself equal with God”.
Therefore, by considering the primary definitions of God, and seeing that Jesus fits every one of those definitions, obviously, Jesus must be eternal God.
References to Trinity in Bible
The Old Testament was written long before the existence of the Christian church, apostate or true, and it teaches there are three persons in the Godhead in Isaiah:
Isaiah 48:16, 17 Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; From the time that it was, there am I: And now the Lord God, and his Spirit, hath sent me.
“I was watching in the night visions, And behold, One like the Son of Man, Coming with the clouds of heaven! He came to the Ancient of Days, And they brought Him near before Him” (Daniel 7:13).
The Son of man, Jesus, is seen coming before the Ancient of Days—who is, obviously, God the Father.
Paul frequently referred to the three separate persons of the Godhead.
“The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all” (2 Corinthians 13:14).
“How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?” (Hebrews 9:14).
We clearly see three distinct persons at the baptism of Jesus.
“And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased” (Matthew 3:16, 17).
Here God the Father bears witness to God the Son through God the Holy Spirit in the form of a dove. And on top of this, it is through the shared authority of these three persons that we are commissioned to baptize.
“Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” (Matthew 28:19).
In John 16:12-15, Jesus gives in His own words gives a glimpse of the Trinity:
"I have much more to tell you, but now it would be too much for you to bear. When however the Spirit comes who reveals the truth about God he will lead you into all truth. He will not speak on his own authority, but he will speak of what he hears and will tell you of things to come. He will give me glory, because he will take what I say and tell it to you. All that my Father has is mine; that is why I said that the Spirit will take what I give him and tell it to you."
Paul gives us the idea of Trinity in this verse:
Romans 15:30 . Now I beseech you, brethren, for the Lord Jesus Christ's sake, and for the love of the Spirit, that ye strive together with me in [your] prayers to God for me;
Godhead of Holy Spirit as Jesus explained here:
Matt 12:32 Whoever speaks a word against the Son of Man will be forgiven. But whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
How they are different yet One?
Again to quote from the Athanasian Creed:
Just as we are compelled by the Christian truth to acknowledge each distinct person as God and Lord, so also are we prohibited by the catholic religion to say that there are three Gods or Lords.
The Father is not made nor created nor begotten by anyone.
The Son is neither made nor created, but begotten of the Father alone.
The Holy Spirit is of the Father and of the Son, neither made nor created nor begotten, but proceeding.
Thus, there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits.
And in this Trinity none is before after another; none is greater or less than another;
But the whole three persons are coeternal with each other and coequal, so that in all things, as has been stated above, the Trinity in Unity and Unity in Trinity is to be worshiped.
It would be pompous and preposterous to pretend that we understand everything about God.
“O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out!” (Romans 11:33).
If we could completely unpack Him like cracking some genetic code, He would cease to be God. John Wesley said, “Bring me a worm that can comprehend a man, and then I will show you a man that can comprehend the triune God!”
Nevertheless, there is much about God that is revealed for our blessing.
“The secret things belong unto the Lord our God: but those things which are revealed belong unto us and to our children for ever” (Deuteronomy 29:29).
What is revealed is that this teaching of the trinity must be important to God. The ministry of Jesus both begins and ends with an emphasis on the three persons in the Godhead. The Father, Son, and Spirit are present at Jesus’ baptism and when He ascends to heaven. Jesus commanded His followers to baptize in the name of Father, Son, and Holy Spirit.
The testimony of Scripture indicates that the Godhead can neither be separated into three Gods nor merged into one person.