From a reader, who wishes to remain anonymous for obvious reasons:
My son attends a Catholic high school. Last year, when he was a freshman, his history teacher offered extra credit to a student to to read this book. I found that appalling, and let him know it. Even though the book is "fiction", I have several coworkers who believe that much of the historical information in this book is true. Apparently, the author gives that impression. Catholics, and nonCatholics, are so ignorant of Church history that they just fall for anything.
Quite true. After all, the Chicago Tribune said Dan Brown's fourth novel is "brain candy" that "trans[mits] several doctorates' worth of fascinating history and learned speculation." The Library Journal opines that TDVC is "a compelling blend of history and page-turning suspense." And BookReporter.com states:
A stunning new thriller that will provoke much debate. Dan Brown's extensive research on secret societies and symbology adds intellectual depth to this page-turning thriller. His surprising revelations on Da Vinci's penchant for hiding codes in his paintings will lead the reader to search out renowned artistic icons as The Mona Lisa, The Madonna of the Rocks and The Last Supper. The Last Supper holds the most astonishing coded secrets of all and, after reading The Da Vinci Code, you will never see this famous painting in quite the same way again.
Well, we know a lot more about that "extensive research" now, thanks to the trial in London. Brown's reliance on Holy Blood, Holy Grail is obvious (regardless of whether or not it is plagiarism), as is his admitted reliance on The Templar Revelation for nearly everything in the novel about Leonardo da Vinci's life and artwork:
The Templar Revelation discussed secret Templar history and the possible involvement of Leonardo da Vinci. This Da Vinci connection fit well into my desire to write in Langdon's domain, the world of art. I became excited about using Leonardo da Vinci as an historical touchstone and plot device for my new novel. Bernini had been central to Angels & Demons and I had enjoyed writing that book. Moreover, I knew Blythe was an enormous fan of Leonardo da Vinci and would be eager to help me research. It was probably at about this time that I came up with the title The Da Vinci Code. ... Da Vinci is also the connection between art and the secret society that I chose to include in The Da Vinci Code - the Priory of Sion. Like Da Vinci's paintings, the Priory of Sion and Da Vinci's involvement with it is discussed in Templar Revelation. (Witness Statement, pars. 97, 102)
But if there are still those who think the novel is "just a novel," please take the time to read my article, "The 'It's Just Fiction!' Doctrine: Reading Too Little Into The Da Vinci Code" (March, 2005. IgnatiusInsight.com), which explain why that argument/cliche simply doesn't work.