Jesus spoke of God the Father, the Holy Spirit and himself as the Son of God in numerous places within the gospels of the New Testament.
First of all, the New Testament was added after the book of Deuteronomy, so you do accept writings that came after the first admonition to not add to the words.
That is fine, I accept them also.
Now, that Jesus spoke of three persons as being divine does not say he taught the doctrine of the Trinity. I have the scriptures on my computer and I did a word search using Trinity as a Key. The search came up with no instances of the word appearing in the Bible.
I posted a quotation from the Catholic Encyclopedia showing what the teaching of the Apostolic Christian Church was.
The teaching of the trinity is certainly different than that.
The Old Testament also teaches that the Second Coming will come after there is a falling away. The change in the doctrine from what it was in the Apostolic Christian Church is certainly a falling away from that truth.
If it isn't a falling away, explain how the change took place.
“Before Abraham was, I Am” ... Jesus defined Himself to be God and when He was about to leave He told the disciples He would send to them The Holy Spirit. ONLY God can make such claims, not ‘a god’, THE God.