“That some Pontififfs have fallen into heresy, some have tried to prove it, but have never proven it, nor will they ever prove it; and we will clearly prove the opposite in the end of Chapter X. Moreover, if God allowed, that a Pope was notoriously heretical and contumacious, he would cease to be Pope, and he would vacate the Pontificate. But if it were a hidden heretic, and did not propose any false dogma to the Church, then no harm would come to the Church: but we must rightly presume, as Cardinal Bellarmine says, that God will never allow any of the Roman Pontiffs, even as private men, become a heretic, notorious, or hidden.”
(Verità della Fede, p. 455, cited in Kramer, Paul. To deceive the elect: The catholic doctrine on the question of a heretical Pope . Kindle Edition.)
To those who hold Opinion No. 3, 4, or 5 of the Five Opinions expounded upon by St. Robert Bellarmine regarding the pope and heresy, you oppose St. Alphonsus Liguori.