What is the theology behind those on this forum who hold that Francis is a valid pope? I noticed that Sedevacantists explained their position very well, but have not so far read an argument from the Sedeplenists on this question.
Perhaps someone could explain it for me.
It is actually quite simple. Since there is no tribunal within the Church with the right to pass judgment against him, he cannot be removed from his office, even though he be under censure, and,
according to the law, have no right to function as the head of the Church. We, his subjects, are not permitted to do anything about this. It is not within our right to declare his acts devoid of validity, due to his having been expelled from his office.
Yes, the faithful may know well that he has committed a sin to which a censure
is affixed by the Church,
but this knowledge in no way qualifies them to declare him deprived of his office, or never to have been elected, regardless of the position taken by proponents of the SV opinion.
While it may be true that some of the Fathers and some papal teachings have taught that any cleric who is or was known to have deviated from the faith cannot hold any office including the office of Pope - it is likewise true that we are taught exactly what it is that we can do about it - and declaring him to not be the pope is not it.
As Fr. Wathen puts it:
.....However, even though the hierarchy cannot take legal action against an heretical pope, all of them together, or any one of them in particular, can condemn his teaching; they can accuse him before God's tribunal, warn him of his sins, and remind him of the divine wrath. Should this measure fail to produce any correction, they can denounce him before his subjects, the Catholic faithful, and warn them that they are not to listen to his teaching. Indeed, not only may the prelates of the Church do this, they have a most serious obligation to do it, an obligation which is as grave as the heresies are pernicious and scandalous. And if they fail to do this, they become a party to the pope's crimes, and will most certainly share in his punishment.
Moreover, where the bishops default in their solemn duty to protect the Church and God's Little Sheep, the priests and the laypeople have not the right, but the duty, to raise their voices against an heretical pontiff. They not only raise their voices to God in prayer for the misguided man, but they also speak out to the bishops and the priests, and among themselves so as to warn their brothers and sisters in Christ that the plague of heresy has infected even their Holy Father, and has rendered him dangerous and unclean. - From his book: Who Shall Ascend?