Having been a reader of Michael Baigent's books, I already understood this to be true long before anyone ever heard of Dan Brown.
And the main point I was trying to make still has some truth to it, and that was, the First Council at Nicea, resulted in the Nicene Creed, which was intended to completely eliminate any theological diversity in Christianity.
"We believe in one God, the Father Almighty, Maker of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, begotten of the Father the only-begotten; that is, of the essence of the Father, God of God, Light of Light, very God of very God, begotten, not made, consubstantial with the Father; By whom all things were made both in heaven and on earth; Who for us men, and for our salvation, came down and was incarnate and was made man; He suffered, and the third day he rose again, ascended into heaven; From thence he shall come to judge the quick and the dead. And in the Holy Ghost. But those who say: 'There was a time when he was not;' and 'He was not before he was made;' and 'He was made out of nothing,' or 'He is of another substance' or 'essence,' or 'The Son of God is created,' or 'changeable,' or 'alterable'— they are condemned by the holy catholic and apostolic Church.
The first of the highlighted passages above is clearly a declaration of the divinity of Jesus Christ, from birth, and
not from the resurrection, and the second higlighted passage is a clear and obvious warning to those who don't follow the first highlighted passage.
While they did not make any rulings about what books would be canonized to become the Bible, they certainly formalised the
removal of any books that contradicted the idea that Jesus was always divine, from his birth, as well as those problematic Gnostic ideas that eternal salvation could be achieved without the church.