If he was “controversial”, it was because he exposed the influence of Satan in the Catholic Church and that many people in power had sold their souls (and that of their children) to Satan. Listen to his interviews with Art Bell from awhile back. Scary and eye-opening stuff.
The strategy is to use the book as a vehicle to engage in some unflattering historical revision. I read half of the book, but it was so far out from anything I am familiar with, I never finished it. It is profoundly disturbing.
I heard Martin interviewed a few times and found him to be brilliant. He was very critical of the Catholic church but very loyal to it.
I’ve read several of his books and never found any reason to question Martin’s veracity.