Gary Giuffre: Before that happens, all attempts to defend any shred of legitimacy for John XXIII, Paul VI, Vatican II or the changes since 1958, John Paul I, John Paul II, or Benedict XVI, cause those who engage in such mental gymnastics, at least by implication, to deny the indefectibility and infallibility of the Church itself. Similarly, those who hold that a legitimate conclave, conducted by the papal princes of the Church to whom is guaranteed the guidance of the Holy Ghost in the selection of the rightful Vicar of Christ, somehow elected an antipope, also deny the indefectibility of the Church.

But aren't you a sedevacantist?  If you're promoting the applicability of "Universal Acceptance" in this case, than that would militate against sedevacantism in general.

I don't believe that Universal Acceptance can unseat a legitimate pope.  There's a historical precedent.  I forget the names of the Popes involved (was it Martin?), but one pope was taken into exile, and another was elected (and universally accepted) while the former was still alive.  That's the equivalent of the Church unseating a Pope, which cannot happen.

Well, I disagree with Bellarmine.  That would contradict the principle that the Church cannot depose a legitimate pope.

Next, two years after the fall of Liberius, concerning which we spoke above, then the Roman Clergy abrogated Liberius from the pontifical dignity and conferred it upon Felix, whom they knew to be Catholic. From that time Felix began to be a true pope. Although Liberius was not a heretic , still it was considered that, on account of the peace made with the Arians, that he was a heretic, and from that presumption, his pontificate could rightly be abrogated. For, men are not held (or cannot be held) to thoroughly search hearts; yet when they see one who is a heretic by his external works, then they judge simply and condemn him as a heretic. Jerome shows this in his Chronicle, when he says that many from the Roman Clergy perjured themselves and went to Felix. They are said to have perjured themselves, because they did not keep the oath that they had taken to not receive another pontiff. 
 
Next, Felix, now a true pope, noticing the danger to the Church and the faith, without a doubt inspired by God, who did not desert his Church, not only receded from communication with the Arians, but even compelled a council and declared the Emperor Constantius, as well as the bishops Ursacius and Valens, with whom LIberius had made peace, to be truly heretics. And for that reason, when Liberius returned to the city, Felix was ejected with his own by the Arians, and died not long after, whether beheaded, or consumed in labours.  That is not known for certain.  This, however, bears on the matter that Felix, after the fall of Liberius, was a true pope, and died for the Catholic faith, which is proved by these arguments."

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I think the principle is that the Church cannot depose a certainly legitimate pope.

Well, if Siri had been elected Pope, had accepted, and chosen the name Gregory XVII, as has been claimed, then he would have at that moment been "certainly" the Pope.  Theologians agree that he becomes the Pope at the very moment of his acceptance of a legitimate election.  He doesn't only become the Pope upon walking out onto the balcony and being revealed to the world.

But aren't you a sedevacantist?  If you're promoting the applicability of "Universal Acceptance" in this case, then that would militate against sedevacantism even more.

I don't believe that Universal Acceptance can unseat a legitimate pope.

There's a historical precedent.  I forget the names of the Popes involved (was it Martin?), but one pope was taken into exile, and another was elected (and universally accepted) while the former was still alive.  That's the equivalent of the Church unseating a Pope, which cannot happen.

That's another reason why the Siri Theory is, theologically, a more satisfactory explanation for the Crisis.  Had there not been a legitimate pope impeding the election of Roncalli, then one might argue that Unviersal Acceptance of Roncalli ensured his legitimacy.