The concept of the Trinity is present from the opening verse of the Bible, actually. The word "God" in Genesis 1:1 is "elohim." This is not a simple plural of the word 'god.' The plural of that word, which means 'two,' is "eloh." "Elohim" means "three or more."
In Deuteronomy 6:4, we have the resounding,
"Hear O Israel, the LORD our God, the LORD is one!"
"God" is, there again, "Elohim." What is also interesting is that the last word of that, the word "one" is the word "echad." "Echad" means unity in plurality. It is the same word used regarding marriage in Genesis. 2:24, when a man is to leave his mother and father and become one with his wife. The word which is NOT used there to mean "one" is "yachid." "Yachid" means a unique singularity.
Now go to Isaiah 9 -- the famous Christmas verse:
"For unto us a child is born
to us a son is given,
and the government will be on his shoulders.
And he will be called
Wonderful Counselor,
Mighty God,
Everlasting Father,
Prince of Peace."
Now go to Isaiah 44:6 --
"This is what the LORD says -- Israel's King and Redeemer, the LORD Almighty:
I am the first and I am the last;
Apart from me there is no God."
Please cross reference this with Jesus' words to John in Revelation 1:17-18 --
"Do not be afraid.
I am the First and the Last.
I am the Living One.
I was dead, and behold I am alive for ever and ever."
Check the Gospel of John, opening sentences, opening chapter:
"In the beginning was the Word. And the Word was with God and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made. [Remember Genesis 1:1 -- "In the beginning, God created the heavens and the earth.:] .... The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth."
Please note also that the absolutely correct translation of the Greek is "...and God was the Word," - I urge you to look it up.
And remember Jesus words' at the end of Matthew: "...baptizing them in the name of the Father, and of the Son, and of the Holy Spirit...." and, again, Jesus' words defining eternal life in John 17:3 clearly equate Him with the Father.
And so, although we may not understand the Trinity with our human minds very well, the doctrine of the Trinity is present in the Bible from the first. Jesus is God Himself in the flesh, and it was because this was His very claim that the Pharisees were so outraged and attempted several times to stone Him.
Again the Jews picked up stones to stone him, but Jesus said to them, "I have shown you many great miracles from the Father. For which of these do you stone me?"
We are not stoning you for any of these," replied the Jews, "but for blaphemy, because you, a mere man, claim to be God "
that is from John 10
Now go back to John 8:24
"if you do not believe that I am the one I claim to be, you will indeed die in your sins."
The Greek for the underlined portion there is "ego eimi" or, literally, "I am I AM"
Jesus says He is God. John says Jesus is God. Thomas worshiped Jesus as God and was not rebuked.
I think I will believe the Bible. Jesus is God.