They're both more than simply plots against particular religious groups, I think.
The Da Vinci Code is based on
The Holy Blood and the Holy Grail, which advances a Q-Anon style conspiracy theory to the effect that Jesus and Mary Magdalene were married and had children, and that their descendants emigrated to what is now the south of France, intermarried with various elite families, and eventually founded the
Merovingian dynasty, and we know this because famous people who were in on this left Easter eggs strewn throughout history for the intrepid doing-their-own researcher to discover.
Antisemitism, as Yair Rosenberg argues in the latest issue of
The Atlantic (
Evernote link), differs from most other forms of racism in that it's based not simply on antipathy to towards a particular ethnic group but on an astonishingly flexible conspiracy theory that purports to explain pretty much everything that someone might dislike, which is why it unites so many different and mutually hostile constituencies.
They're not plots, though they might purport to describe plots, so much as pernicious conspiracy theories.