It certainly helped to have read all about that background stuff!
I remember watching the original programme on this in the Chronicle series, The Priest, The Painter and The Devil in 1974, entranced at the pieced together and piecemeal story. As it was before videos or indeed frequent repeats, it was 5 years till it and the follow up appeared again, and then this obviously went onto become The Holy Blood and and The Holy Grail. When this become outed as based on a set of frauds which the writers hadn't really made much effort in investigating, it was fair enough, part of the thrill and an informative example of fake news, or rather fake history.
That all of this received a thorough examination in terms of the possibility of hidden truths and a parody thereof in Foucault's Pendulum by Eco, long before Brown vomited his execrable novels on the unsuspecting world like a lovely horse with fetlocks flowing in the wind and having a bad case of food poisoning, seemed to me to sound the end of such speculative semi histories. How foolish I, the immensely tall reader, was.
The original was just part of a huge swathe of books based on an intention to ignore facts for the sake of sensation from the lunacies of Von Daniken to Graham Hancock to those covered in Them by Jon Ronson such as Alex Jones.
It looks almost funny, with David Icke and his lizards but much of it is tied up in a deluded melange of anti semitism, ethnic tribalism and logical illiteracy. For Brown to come in in his clunky plagiarising fictions and sweep up the witlessness of misunderstandings could be seen as a genius of opportunity, even if the prose and ideas are valueless retreads.