Ubertino of Casale in his work Arbor Vitae on the impossibility of papal resignation, and the Antichrists installed after Celestine V, an excerpt:
"...This is a great mystery. But I say, in Christ and in the Church: even if one should say that the carnal union of a man and his wife is a great union, nevertheless it is incomparably greater between Christ and the Church. And by this the same is true between the Supreme Pontiff and the whole Church, since he is its Christ. With this agrees expressly the decretal of Innocent Extra, De translatione episcopi, inter corpora.
The falsity of the second principle—which says that anyone may renounce his right on account of the stupidity of the allegation—is not worthy of mention. For the papal authority is not a human right, but that of the Most High God, upon whose grant and deprivation it depends. This is clear in the published decretal, in which it is said that a particular bishop cannot be separated from his own church except by the Lord; and since this is done by the Supreme Pontiff, who acts in the place of God, it ought not to be said to be done except by God, whose authority he exercises. Nor is it lawful for any bishop, by his own authority, to separate himself from his church. Here, to the point at hand, it is clear that the right of a bishop is not in the power of the bishop himself, nor is a church in the power of its own church, nor in the power of a superior archbishop, but only in the power of Him who exercises the plenitude of the vicariate of God.
O what an express and unbreakable argument this is for the principal intention: that it is impossible for the Supreme Pontiff to resign! For who is there upon earth who acts in the place of God over him, so as to absolve him from the sacramental union of the spousal bond with Christ, to whom Christ has immovably bound her? Certainly only death divides this bond, which the Supreme Pontiff receives from the principal Head, Jesus—as it is fitting that one member should be united with the others. As a sign of this inseparable union even unto death, when Jesus had said to Peter, “Feed, feed, feed,” He prefixed for him the martyrdom of his death; and afterwards He said, “Follow me.” Which means: to follow me is to feed the sheep solicitously, and never to abandon them for any necessity whatsoever, but to endure martyrdom for them.
The right of the pontiff, therefore, is not of man but of God—unless perhaps it was procured by ambition, which is not for the salvation of souls but for the ambition of his own heart, through fraudulent malice.
But let us return to the text of the Apostle which I was expounding. You have heard of the three evil and false signs which this Beast, which harms in many places, caused the false prophet to make through false sciences and his own appearance, and through his own evil seduction, in favor of the body of the Beast before the world. It follows: “And it was given to him to give breath to the image of the Beast, that the image of the Beast should also speak.” This was granted, at the instigation of the devil and by the permission of God’s judgment, to this false prophet and to the Beast rising from the earth, so that the authority of him who, through me the Beast, was in reality lifeless and without the life of divine right, might appear living and spiritual; and as though speaking rightly and canonically introduced, and so that in her the final power of loosing and binding might be exercised as if from the true Pope—as if the image of the Beast itself were speaking. And what it says—that it causes all, small and great, rich and poor, free and slave, to have the mark of the Beast on their right hand or on their foreheads, and that no one may buy or sell unless he has the mark, that is, the name of the Beast or the number of its name—this is to carry the mark of the Beast on the forehead, or the name, or the number of the name: to manifestly confess its power. To carry the aforesaid things on the right hand is to fulfill efficaciously what it commands. But to be unable to buy or sell without these things means to be unable to obtain or confer benefices in the Church, and to be unable to perform spiritual commerce—preaching, reading, sermonizing, ministering—without them.
We have seen this fulfilled to the letter, and efficaciously preached by this Beast. Certainly at that time there arose pseudo-Christs (that is, false pontiffs) and pseudo-prophets, false religious, and they gave the great signs predicted and the signs themselves. For bishops, archbishops, cardinals, and many other prelates and religious, pseudo-clerics, were deposed and slain with the spiritual sword and put to corporal death by other punishments, because they refused to bear the aforesaid marks. And what is worse: even though they were suspected of this, even if they bore the evil mark, they were so ready to do evil against men of virtue that, for the price of the Beast, their own status lay in danger. Many—and we have seen almost all the prelates of the Curia marked with the sign of the Beast—publicly serving this [Beast] against the champion of truth, consigning false crosses with which the pseudo-prophets sent throughout the world by the Beast marked them."