For fiction, Mr. Brown claims it is fact.
From the official web site:
FACT:
The Priory of Sion—a European secret society founded in
1099—is a real organization. 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, Botticelli, Victor Hugo,
and Leonardo da Vinci.
The Vatican prelature known as Opus Dei is a deeply devout
Catholic group that has been the topic of recent
controversy due to reports of brain-washing, coercion, and
a practice known as "corporal mortification." Opus Dei has
just completed construction of a $47 million National
Headquarters at 243 Lexington Avenue in New York City.
All descriptions of artwork, architecture, documents, and
secret rituals in this novel are accurate.
PROLOGUE
http://www.danbrown.com/novels/davinci_code/excerpt.html> All descriptions of artwork, architecture, documents, and secret rituals in this novel are accurate.And they most assuredly are not. In fact it's been noted that as editions come out, more corrections are made.
Not long ago I found a website where actual Parisians were pointing out all the erroris in the dscriptions of architecture, documents and streets in the book. I've already had it with arguing over how much art history this book flubs.
None of these were behind my reasoning for abandoning the book, btw. I just couldn't take one more cliched, uninteresting second of the stupid thing.