Of course, the characters and plot are fictional, I believe it is obvious that Dan Brown believes, and claims truth, for a great deal of the 'historical' material in the book.
Some excerpts attributed to Brown from his website illustrate:
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.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.
Since the beginning of recorded time, history has been written by the "winners" (those societies and belief systems that conquered and survived). Despite an obvious bias in this accounting method, we still measure the "historical accuracy" of a given concept by examining how well it concurs with our existing historical record. Many historians now believe (as do I) that in gauging the historical accuracy of a given concept, we should first ask ourselves a far deeper question: How historically accurate is history itself?
The ideas in this novel have been around for centuries; they are not my own. Admittedly, this may be the first time these ideas have been written about within the context of a popular thriller, but the information is anything but new.
Two thousand years ago, we lived in a world of Gods and Goddesses. Today, we live in a world solely of Gods. Women in most cultures have been stripped of their spiritual power. The novel touches on questions of how and why this shift occurred…and on what lessons we might learn from it regarding our future.
Revealing that secret would rob readers of all the fun, but I will say that it relates to one of the most famous histories of all time…a legend familiar to all of us. Rumors of this conspiracy have been whispered for centuries in countless languages, including the languages of art, music, and literature. Some of the most dramatic evidence can be found in the paintings of Leonardo Da Vinci, which seem to overflow with mystifying symbolism, anomalies, and codes. Art historians agree that Da Vinci's paintings contain hidden levels of meaning that go well beneath the surface of the paint. Many scholars believe his work intentionally provides clues to a powerful secret…a secret that remains protected to this day by a clandestine brotherhood of which Da Vinci was a member.
I first learned of the mysteries hidden in Da Vinci's paintings while I was studying art history at the University of Seville in Spain. Years later, while researching Angels & Demons and the Vatican Secret Archives, I encountered the Da Vinci enigma yet again. I arranged a trip to the Louvre Museum where I was fortunate enough to view the originals of some of Da Vinci's most famous works as well as discuss them with an art historian who helped me better understand the mystery behind their surprising anomalies. From then on, I was captivated. I spent a year doing research before writing The Da Vinci Code.
Most of the information is not as "inside" as it seems. The secret described in the novel has been chronicled for centuries, so there are thousands of sources to draw from. In addition, I was surprised how eager historians were to share their expertise with me. One academic told me her enthusiasm for The Da Vinci Code was based in part on her hope that "this ancient mystery would be unveiled to a wider audience."
"A prankster and genius, Leonardo da Vinci is widely believed to have hidden secret messages within much of his artwork. Most scholars agree that even Da Vinci's most famous pieces—works like The Mona Lisa, The Last Supper, and Madonna of the Rocks—contain startling anomalies that all seem to be whispering the same cryptic message…a message that hints at a shocking historical secret which allegedly has been guarded since 1099 by a European secret society known as the Priory of Sion. In 1975, Paris's Bibliothèque Nationale discovered parchments known as Les Dossiers Secrets, identifying numerous members of the Priory of Sion, including Sir Isaac Newton, Victor Hugo, Botticelli, and Leonardo da Vinci.All of this is woven up in DVC. Brown believes it is real - and claims truth for it.