I am convinced that Dan Brown is knowingly deceiving millions. I cannot possibly believe that the "pain-staking" research he claims was ever done..... unless all of the problems to his theory were purposely ignored.
For instance, the integrity of the DaVinci Code hinges on what he calls the "fact" that Leonardo DaVinci was the grandmaster of the Priory of Sion at one point. The Priory did not exist until 1956, and has nothing to do with protecting esoteric secrets about Jesus. Therefore there was nothing for DaVinci to encode, and the motivations Brown attributes to him cannot exist.
The BBC exposed the Priory of Sion hoax in 1996. When you want to do "pain-staking" research on a topic, how do you miss a BBC investigation? How?
The idea that Mary Magdalene was portrayed in DaVinci's Last Supper is absolutely laughable for art historians. This was exemplified in the ABC special when the woman interviewer brought the art historian to view the painting, and asked him, in all seriousness, "Doesn't that look like Mary Magdalene?" The look on the professor's face was priceless! It was one of those looks when someone has asked a question so incredibly stupid that you actually lose respect for them on the spot!
How could Brown not, in his painstaking research, have come across Leonardo's notes and sketches of the painting... in which all of the apostles are labeled by name?
And when he claims that at the Council of Nicea, Constantine (a) deified Jesus, and (b) established the canon of the New Testament, why did he not read the resolutions of the Council? They are readily available to anyone with an internet connection. The Council said nothing on the divinity of Jesus (this was well established in the New Testament, not to mention various writings of the early theologians) and had nothing to do with the formation of the canon (this would not happen, for good, for another ~65 years at Hippo).
I have lost all respect for Dan Brown, except the respect due to him by the mere fact that he is alive and a fellow human. He is one of those who you should "love as a neighbor, but not like," to paraphrase C.S. Lewis.