Actually, here's some excerpts from an interview on his website;
HOW MUCH OF THIS NOVEL IS TRUE?
The Da Vinci Code is a novel and therefore a work of fiction. While the book's characters and their actions are obviously not real, the artwork, architecture, documents, and secret rituals depicted in this novel all exist (for example, Leonardo Da Vinci's paintings, the Gnostic Gospels, Hieros Gamos, etc.). These real elements are interpreted and debated by fictional characters. While it is my belief that some of the theories discussed by these characters may have merit, each individual reader must explore these characters' viewpoints and come to his or her own interpretations. My hope in writing this novel was that the story would serve as a catalyst and a springboard for people to discuss the important topics of faith, religion, and history.
BUT DOESN'T THE NOVEL'S "FACT" PAGE CLAIM THAT EVERY SINGLE WORD IN THIS NOVEL IS HISTORICAL FACT?
If you read the "FACT" page, you will see it clearly states that the documents, rituals, organization, artwork, and architecture in the novel all exist. The "FACT" page makes no statement whatsoever about any of the ancient theories discussed by fictional characters. Interpreting those ideas is left to the reader.
Here's what happened I think. On the first page he writes that the book contains FACTS in it, but he really leaves it a bit ambigious so that the reader will question what is FACT and fiction as he/she reads. It's simply a brilliant way to sell books if you ask me, not a plot to destroy Christianity or defame Jesus. A careful reader will interpret responsibly what he says and conclude that he is talking about what is real is FACT.
The first page reads,
All descriptions of artwork, architecture, documents, and secret rituals in this novel are accurate.This is his way of letting the reader know that when he/she comes across a reference to a church, an art musuem, or an artwork, that D. Brown has done his homework. So in reading the novel one has not only been entertained, but somewhat educated.