I first read this as a credulous 15-year-old, but even then I found the claims to be entirely unsupported by the evidence cited. In the years since then, I am amazed that I could have been that generous.
This is a classic case of conspiracy theory and pseudo history: cite a few plausible assertions as proven fact, and then pile loads of implausible claims and speculations on top of those supposed proven facts. It makes for a gripping narrative, but rapidly departs from what we know as reality.
This is not to say that this isn't an extremely entertaining book, but only if you approach it entirely as a work of fiction. It is a fantastic yarn, and lots of fun, but please don't take it seriously at all.
and . .
"The book's reasoning is based mostly on 2 presumptions: a poor priest has found something in Rennes-le-Chateau, and this something is a proof that Priory of Sion has some secret knowledge. Both of them are just eleborate hoaxes - everywhere in Internet you can find who, when and why concocted them (just look, for example for "Rennes-le-Chateau hoax". Priory of Sion was just a club of friends, some of them wrote the "ancient" parchments, and Mr. Plantard has never been a "St.Clair", and priest Sauniere received his wealth from illegal selling of masses, and died in poverty, after Church investigation. I recommend reading "The Unknown Treasure: the Priory of Sion Fraud and the Spiritual Treasure of Rennes-le-Château"
finally . . .
"I first read this garbage over 30 years ago and it is very depressing to see it make a comeback. Knowing someone in the supposed Priory of Sion makes me even less convinced - this really is gobbledygook of prime order."
Hint: I wouldn't stake my eternity on it.