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Okay, so reading the article, he's saying that this argument really applies to Paul VI in a way that doesn't really fit his successors. The problem is that Paul VI appeared to be elected by a legitimate council, by legitimate cardinals, etc. And people didn't really know who he was when he was elected, and so on. So he was accepted as a true pope by most Catholics, until Vatican 2 and the new Mass starting coming out, and people started to wonder what was going on. So, the answer was that Paul VI had been a true pope but had lost the papacy, since they thought he had been a real pope to begin with.
The difference between Paul VI and his successors is that sedevacantists understood the situation by the time Paul VI died, and didn't accept subsequent conclaves as legitimate, and presumed that JP2 and so on were modernists and part of the new Church. The concept of a "new Church" didn't exist when Paul VI was elected (arguably the "new Church" didn't even exist at that point anyway), but by the time he died in 1978 it was clear it was a false religion.
That's how I understand his analysis.
OK. It just doesn't make sense to me. If they were just gravitating toward St. Robert's pious opinion, I'm not really sure why that's necessary, since St. Robert, while favoring that, admitted that it's not impossible that a pope could FALL from the papacy.
I find it problematic to say that, well, later we realized Montini was a manifest heretic, but then came to the conclusion that he must have been a manifest heretic all along. Well, it doesn't work that way. If therre's no evidence of heresy prior to his election, then, no, he was NOT a manifest heretic, by definiton of the term. At best you'd be projecting backwards that he would have been an occult heretic all along (which is quite possible), but occult heresy doesn't remove someone from membership in the Church and therefore from election to the papacy. But of course it's pure speculation, since the heresy would have been occult.
I think they would have done better to just stick with his falling from the papacy.