He presents his work of fiction as historical fact based on the theme or his theory about the Holy Grail.
Do you know if Constantine was an actual Christian and produced an empire based on his beliefs or did he use it as a way to unite the newly conquered people? Did his first high priests not only come up with the idea of the Trinity but also many of the Catholic doctrines (to keep people in line)? Was he the one responsible for the supposed cutting and pasting of the scriptures to meet his controlling needs? Brown puts down the latters as fact.
According to Brown,
The Knights of Templar are thought to have been banished or went into hiding but he displays them as the protectors of the Grail (the Grail being information) until present day where a secret society kept it hidden until the world was 'ready' to hear the 'truth' about Jesus Christ.
Da Vinci, a shameless pagan, received contracts from the early Christian churches to paint religious murals and large portraits. He was a prankster and he would hide little pagan symbols in the paintings to show his disdain. You can look up the particular paintings on the internet and actually see what he's talking about. One of them is none other than The Last Supper, showing that one of the disciples, sitting closest to him, is a female.
Mary Magdelene was the most important disciple, true. Jesus may have been the first feminist in history. Brown puts her on this pedestal because she was the wife of Jesus and they were very affectionate towards eachother, publicly. She was pregnant with His daughter, Sarah, at the time of His crucifixion. Mary M. and mother Mary were the only ones who hung around-remember the rest of the disciples fled. Then Brown says Mary M. fled to France after the death of Jesus and gave birth. There were a few names the main character new of that might have been part of Jesus' bloodline, after the fact. At the end, you get to find out who His present-day descendants are. No one famous, and not really a surprise.
So, the blaspheme goes on and on. Still wanna read it?