I never hoped to live to see the day, where the DaVinci Code was cited as anywhere near a credible source.
Go look up the wikipedia article about it, I seem to recall it listing a good page or so of inaccuracies, misrepresentations, or outright sophistry.
In fact the biggest fallacy he presents, is the claim that work is based on any sort of credible research or hitherto-unexplored source, and that it is to be taken as factual.
Now, sure, it may accidentally stumble upon some cogent points, such as the common knowledge that Christmas wasn't the date of Jesus' birth, or that certain symbols have persisted or been syncretized by religion, but I would far rather you look up the cites listed by Jon`C et al if you want a decent exploration of such topics.
Also, it would be great folly to pretend that sort of thing is anywhere near unique to Christianity. For starters, look at the Romans, who more or less stole their entire Pantheon [because they had pants gods, like Geoff, the god of biscuits] from the Greeks.
For that matter, there is a fair bit of reverse syncretism with regards to Christianity and other religions. Taking the Roman example again, how about the replacement of Mithra with Christ as their new God of war [and the Xi-Rho shield marking, Xp], without a resultant immediate adoption of the Christian beliefs themselves. It was also very common for pagan religions to adopt Jesus, or particularly Mary, as another deity in their pantheon, or simply rebuild a religion around one of them [as per the Cult of Mary].
Take also South American Catholicism, where a great many indigenous beliefs/celebrations were retained.
For that matter, one could look at Zeus Ammon as a direct adoption of the Egyptian sun god Amun, brought back by Alexander the Great from Siwa. Or that it it vser likely Mercury and Hercules were Odin and Thor in disguise.
Here also, one could look at the evolution of the Buddha in different areas. Much like Christ, he tended to be depicted in a very localized fashion. [in fact I seem to recall one classical mosaic where Christ is shown with a Roman bowl-cut]. However, this becomes particularly noticeable as he approaches China and Japan, passing through areas where earth-mother goddess worship had been in place at various times. The large rotund Buddha seems to have effectively taken the form of the earth-mother goddess [compare with the Venus of Willendorf]. Much like Christianity and the Cult of Mary, Buddha developed a female counterpart known as Kwan-Yin [Guan-Yi sometimes, pardon spelling]. Later on, even, Shinto adopted several aspects of Buddhism as well.
Perhaps it is best to think of religion not as an underpinning, but as an overlay, an organizing and unifying principle, but not necessarily a complete redefinition. As the various barbarian hordes [say them with me: Mongols and Magyars and Vikings, oh my!] of the 900s AD swept across Europe, they almost all at least in part adopted Christianity as they came, to one degree or another. Even a number of the Viking found their mythology so incredibly depressing that they found orthodox Catholicism to be downright exciting. [Beowulf is a fascinating treatment of the shifting from pagan to Christian beleifs in that respect]. Each of them tended to manifest Christianity in a certain polarized culturally influenced fashion. That's how these things happen.
It would be impossible to discuss syncretism without a nod to the Catholic canon of patron saints, 5001 strong and rising:
http://www.catholic-forum.com/saints/patronnf.htm
It would be very surprising, were more than say 20% of those were actual real extant people, least of all Catholic. Catholicism had this very clever way of gaining an in, of common ground with peoples. Very often they would adopt folk heroes, mythological figures, legendary nobility, and turn them into saints. This not only gave converted cultures a feeling of familiarity and connection, with the adoration of the saints they could often go on worshipping their same deities in their slightly veiled form.
So, I guess I am disagreeing with the underlying sentiment that syncretism [adopting and adapting] is somehow a bad thing or dishonest underhanded unoriginal whatever. When in fact, it is endemic and even outright necessary for culture religion philosopy &c to propegate and survive. Think biomass. Adapt to the host for viral transmission of ideas and philosophy.
Also, I can kill you with my brain.