And it's even more credible because he seems to mention this in passing and does not put any emphasis on it as being particularly important ... since he doesn't really understand or care about the theological ramifications, viewing the papacy to be similar to any political office.
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I don't see how this makes it any more credible. He mentions it because it suits his agenda, which is to portray the Church as a corrupt and incompetent institution. Telling a story about the leader of an institution getting overthrown by an internal faction right as he is getting elected definitely makes the Church look corrupt and incompetent.
The way books work is that when an author makes a claim, he has to put a footnote providing the source for his claim. This allows people to verify that his claim is true. The more serious a book is, the more footnotes it has. Now, if someone puts in a footnote that cannot be verified, that detracts from the credibility of the book and outright negates the claim attached to that footnote, since it is now not only an unsupported assertion, but an assertion "supported" by a footnote that is either a deception or the result of incompetence, either of which further detract from the assertion that has the footnote.
So, when trads try to track down Williams's footnotes and find they are false, and Williams refuses to provide any explanation for why the footnotes are false, this destroys both Williams's credibility and even more so the claims which he supported with false footnotes.
What makes his claim even more credible is where he adds the detail that Siri took the papal name Gregory XVII. If he was just making something up, where did he get that particular detail. Also, Williams was capitalizing on his credibility due to his background, which makes it unlikely that he'd make something up just to mention it in passing that might compromise his credibility.
Nah, the name "Gregory XVII" had been circulating for years before this book came out, mainly from Gary Giuffre and Malachi Martin, who likely stole it from Giuffre.
And if you think Paul Williams was a man who had any concern about his credibility, please just read some of this
ridiculous book we're talking about. It's literally a kitchen-sink of copy-pasta of urban legends and lies against the Church. It's about on the level of Maria Monk. Just spend a few minutes reading random parts of that book; seriously it won't take you long to get a feel for it and for its author.