Well, here is another example of a saint of the Catholic Church claiming that the Popes can and have been heretics, and challenges the modern Roman Catholic conception of the Papacy.
"The school of these heretics during the succession of such bishops, continued to acquire strength and augmentation, from the fact that Zephyrinus and Callistus helped them to prevail. Never at any time, however, have we been guilty of collusion with them; but we have frequently offered them opposition, and have refuted them, and have forced them reluctantly to acknowledge the truth. And they, abashed and constrained by the truth, have confessed their errors for a short period, but after a little, wallow once again in the same mire." (Refutation of all Heresies 9:2)