If Dan Brown had wanted to be entirely forthcoming, then the “FACT” page might simply have said:

“Some names, dates, and facts have been changed to perplex the innocent.”

When considering the “FACT” page, though, it’s worth noting how surprisingly minimal the factual claims here are. After all, The Da Vinci Code is notorious for its astounding assertions: that Jesus married Mary Magdalen and had children; that the Emperor Constantine made Christianity the official religion of the Roman Empire; that “the Vatican” killed five million witches; that a pope killed the Templars; that the Catholic Church has been ruthlessly covering up the truth for two millennia… and on and on and on. The “Fact” page, however, refers to none of these things.
Instead it makes specific reference only to other matters, about which it says virtually nothing at all: the Priory of Sion, which is said to exist, and Opus Dei, which is said to be a somewhat controversial Catholic organization with a big new building in Manhattan.
In a very broad sense, these two points are indeed true.
From this, a very cautious reader might well conclude two things: that the only true facts in the book are the ones that are mentioned on this page; that the rest of the book – including all its historical and religious claims – makes no claim to be true. (This theory receives some slight support from Mr. Brown’s correct identification – on this page, and only on this page Рof Murray Hill Place as the “national” headquarters of Opus Dei, while elsewhere in the book, it is misidentified as the world headquarters.)
This would be a prudent approach to the book – one that would certainly spare an innocent reader a great deal of confusion. Unfortunately, however, this approach is unsustainable, because even the extraordinarily modest claims on this page, which make it easily the dullest page of the book, are grossly exaggerated: the minimal statements about the Priory of Sion and Opus Dei are not factual, and the sweeping claim of accuracy regarding artworks, architecture, and various other matters is totally unjustified.