I'm pretty much convinced that Brown cribbed his Da Vinci Code from a pulpy 80s book, this one. Annoyingly the authors didn't win their copyright case.
https://en.wikipedia.org/wiki/The_Holy_Blood_and_the_Holy_Gr...
I remember reading it in the 90s and being very put off by the insanity and lack of logic. As it later turned out: The conspiracy theory of HBHG and DVC was in the end invented by a French document forger who in his own forgery seemed to be the last merovingian king/descendent of Christ: