.... I have the impression that those, who say "come down, it's just fiction!" are often the same people, who firmly believe that all these things like the "Prieuré de Sion" really existed as Dan Brown described them.
Interesting point.
To make it short: some folks (interested in history) are irritated, because they see that a lot of people let Dan Brown shape their idea about many things (the Catholic Church, the Louvre, Leonardo da Vinci etc.) So it's more about the gullibility of people, who seem to consider themselves to be very skeptical.
Yes, Brown put forward some interesting ideas, as fiction, though he, apparently, believed them to be 'fact'.
Some of them really got people thinking and studying.
Now, because it is 'only fiction', anyone who criticises the errors, may be considered to be a bit over-the-top.
Yet those same 'only fiction' people may actually believe that the fiction is, or is based in, fact ~ just as Brown said it was.
As this thread illustrates, Brown made some very obvious errors ~ for example, he got the number of panes in the Louvre pyramid wrong, in spite of saying that all the architectural detail was correct.
Based on this, can we believe anything else he says, even though he claims that 99% of it is true?
The real truth is that we should check everything out, for ourselves, anyway, and not rely on the author of adventures stories to provide us with out 'facts'.