Actually, I'm not sure that he really has said that it is a work of fiction. And that's the problem.
"The Da Vinci Code is a novel and therefore a work of fiction." Dan Brown (on his web page)
If you carefully hold the book under a 60 watt bulb and stare intently at its cover, you can see a secret message printed just below the title. It reads -- "A Novel" -- which, when run through the "Webster's Special Edition Da Vinci Code Decoder Book," translates to -- "a work of fiction."
Well he readily admits the characters he made up and their story is fiction. But the underlying story of Jesus and Mary having a child together and the conspiracy to cover it up, Brown has been somewhat clear he believes this theory.
Ahhh - usually when a book is sold as fiction and listed in the fiction section of libraries and bookstores, it's accepted as fiction - Most people don't have a hard time understanding that...