Jesus repeatedly claimed to be God.  Also, in Genesis chapter 1, God refers to Himself several times as We/Our.  The trinity existed even in the first chapter of the Bible even if it wasn't explained.

The Apostle John started his gospel account with these words.

"In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God.  All things came into being through Him, and apart from Him nothing came into being that has come into being.  In Him was life, and the life was the Light of men.  The Light shines in the darkness, and the darkness did not comprehend it." (John 1:1-5)

He then goes on to explain the birth of Jesus.  Jesus is the word.  Jesus was with God and was God.  Jesus created everything.  That is the definition of God.