the da vinci code was probably the defining literary phenomenon of the noughties. everybody was reading it in a way that will probably never happen again now that we have social media and stuff. but despite that, it doesn't seem to have left much of an imprint in cultural memory. compare, for example, the harry potter series which was coming out around the same time and continues to endure despite j.k. rowling's best efforts to alienate her audience. in fact at this point i'm almost a bit embarrassed to admit i read the da vinci code, even though i was in middle school when i did.
this might all just be because it's a schlocky christian conspiracy thriller (i mean a catholic albino monk assassin? really?) that had its moment, but i think there might be some other reasons as well. i'm not exactly sure why he did this for what seems to be a minor selling point, but dan brown staked an oddly huge part of his reputation behind the claim that "besides the plot and my characters, 99% of the historical facts in the book is true". it was in the forward of the book itself and he would always talk about it during media appearances. i seem to recall michael crichton would do something similar with his books as well. maybe they were worried people would feel guilty reading a pure fiction book without learning about history or technology at the same time. maybe dan brown was worried people wouldn't get into the book as much if they didn't think there was a possibility albino catholic monk assassins were out there somewhere on the hunt right now.
as it turns outs of course, it later surfaced that the da vinci code was nowhere close to being as historically accurate as dan brown had portrayed it. this was pretty embarrassing because of how much he had stressed the accuracy, and perhaps getting discredited like that is why the book has so little cultural cachet remaining today. but i don't think there was any malicious intent, i think he genuinely believed it was historically accurate. he had done the research himself, after all, taking nearly all of it from a single non-fiction book from the eighties titled holy blood, holy grail. as everyone knows, if a book is non-fiction, that means it is true. you cannot just make up a bunch of stuff and call it non-fiction, that's not how it works. so, i think dan brown's thought process was "i copied all the history stuff directly out of this non-fiction book so those parts are historically accurate."
apparently he did a faithful job, because the authors of holy blood, holy grail sued him for plagiarism and copyright infringement on the basis that the da vinci code reproduces their alternate history theory almost point-for-point (despite dan brown claiming to have never read it). this was a somewhat questionable move on their part because you can't copyright real history, so if holy blood, holy grail describes actual history as they purported, then dan brown is free to use said history in his fiction book. the court said as much when they threw out the lawsuit, although not before holy blood, holy grail saw a big sales bump from the publicity.
so did the authors of holy blood, holy grail just make it all up? well, not exactly. it turns out a lot of their book is in turn lifted from an even earlier "non-fiction" book published in french, titled l'or de rennes. they supplemented it with additional research in the national library of france, mostly based on a collection of historical documents they found there entitled dossiers secrets d'henri lobineau (the secret files of henri lobineau). the documents concerned themselves with a secret society called the priory of sion that protects the modern descendants of the merovingians, the first french dynasty from the early middle ages. to this, the authors of holy blood made some original contributions based on their "biblical obsessions": the merovingians were also the rightful blood descendants of jesus from his marriage with mary magdalene, and the priory of sion protects them from the catholic church and the pope who are afraid of being usurped by true descendants of jesus. all the extra christian parts regarding the lineage of jesus seem to have been completely made up by the authors, which is unfortunate for dan brown's historical accuracy because he used the "full" christian version as a major part of the da vinci code's "historically accurate" main plot.
that's not all, though: the holy blood authors didn't know this, but those "historical documents" they found in the national library of france? they were actually sophisticated literary forgeries, part of an elaborate scheme to meme an ancient secret society called the "priory of sion" into existence. all of it was orchestrated by a french guy named pierre plantard. plantard was a freemason, avid esotericist, and holder of many eccentric nationalist and monarchist political views. it seems that he cooked up the whole priory of sion hoax in an attempt to gain clout in both esotericist and monarchist circles, by painting himself as both a member of a prestigious ancient secret society and the rightful king of france through claimed descent from the merovingians. yes, it turns out all those genealogical documents about the secret merovingian bloodline all ended with a certain "pierre plantard" as the modern heir.
plantard enlisted his aristocratic writer friends gérard de sède and philippe de chérisey in the conspiracy to create a fake conspiracy. as avant-garde writers and surrealists, the idea of remaking the past and hence potentially influencing the future through literature was too enticing to pass up. like a real-life version of the borges short story tlön, uqbar, orbis tertius, de chérisey forged documents about the priory of sion and the merovingian bloodline and planted them across france, the most famous of which ended up being the dossiers secrets d'henri lobineau in the french national library. the french book that inspired holy blood, l'or de rennes, was actually written by de sède working with plantard to publicize the hoax, based on "evidence" from the planted documents and building also on popular prior conspiracy theories regarding a priest in rennes who discovered some hidden parchments and treasure while renovating a church. it had practically become a shared alternate history literary universe, with each subsequent author/conspirator putting in their own original contributions. however plantard was not exactly pleased with all the jesus bloodline stuff the holy blood authors had piled on, which was a little much for him and also went against the proper "franco-israelist" basis he had wanted. at one point plantard basically said in an interview "look guys, i've never claimed to be a descendent of jesus christ, i'm simply the rightful merovingian king of france prophesied by nostradamus, that's all.”
unfortunately for plantard, his co-conspirators didn't share his grandiose neo-monarchic political ambitions. they cracked and spilled the beans when the pressure got too intense, largely discrediting plantard and the priory of sion. despite that, plantard flailed around for years trying to keep it going, occasionally revising his claims or producing some new dubious document to prove it all. eventually he accidentally got himself mixed up in a very serious political scandal and was forced to admit in court he made everything up, then kept a low profile until his death for fear of inviting additional lawsuits. in light of all the revelations, the authors of holy blood distanced themselves from plantard and insisted that there had been a real priory of sion before he made it up and that there work was still valid. this didn't change a whole lot because no serious historians paid attention to holy blood from the outset anyway, although anthony burgess (author of a clockwork orange) wrote in a contemporaneous review: "It is typical of my unregenerable soul that I can only see this as a marvellous theme for a novel."