Brown is right about one thing (and not much more). In the course of Christian history, few events loom larger than the Council of Nicea in 325. When the newly converted Roman Emperor Constantine called bishops from around the world to present-day Turkey, the church had reached a theological crossroads.
An Alexandrian theologian named Arius had argued for a small minority that Jesus had undoubtedly been a remarkable leader, but he was not God in flesh. Arius was enough of a problem and persuasive enough as a false teacher to justify dealing with the issue in this formal fashion. Actually, more specifically, Arius claimed that Jesus was a created being. This effectively denied His divinity. This was a departure from what the church had understood and accepted from the beginning. It was what the martyrs had been willing for three centuries to die for. It is absurd to say that Constantine decided to make Jesus a God.
In The Da Vinci Code, Brown apparently adopts Arius as his representative for all pre-Nicene Christianity. Referring to the Council of Nicea, Brown claims that "until that moment in history, Jesus was viewed by His followers as a mortal prophet … a great and powerful man, but a man nonetheless."
In reality, early Christians overwhelmingly worshipped Jesus Christ as The Son of God, God in the flesh, their risen Savior and Lord.
And as for the vote: it was 318-2 – a "real close" vote as Dan Brown claims.