The question of whether the Holy See is vacant must be considered from three aspects, that is whether a pope could become an heretic and forfeit his office; what constitutes heresy; and whether any of the conciliar popes can be considered to be heretics within the context of this definition.

The problem which would face the Church if a legitimately reigning pope became an heretic has been discussed in numerous standard works of reference. The solution is provided in the 1913 edition of The Catholic Encyclopedia: "The Pope himself, if notoriously guilty of heresy, would cease to be pope because he would cease to be a member of the Church."(2) Many theologians have discussed the possibility of a pope falling into heresy, and the consensus of their opinion concurs with that of The Catholic Encyclopedia. The Pope must evidently be a Catholic, and if he ceased to be a Catholic he could hardly remain the Vicar of Christ, the head of the Mystical Body. St. Robert Bellarmine taught: "The manifestly heretical pope ceases per se to be pope and head as he ceases per se to be a Christian and member of the Church, and therefore he can be judged and punished by the Church. This is the teaching of all the early Fathers."(3) Saint Robert was, of course, discussing a theoretical possibility, and believed that a pope could not become an heretic and thus could not be deposed, but he also acknowledged that the more common opinion was that the pope could become an heretic, and he was thus willing to discuss what would need to be done if, per impossible, this should happen: "This opinion (that the Pope could not become an heretic) is probable and easily defended . . . Nonetheless, in view of the fact that this is not certain, and that the common opinion is the opposite one, it is useful to examine the solution to this question, within the hypothesis that the Pope can be an heretic."(4)

The great Jesuit theologian, Francisco de Suarez (1548-1617) was also sure that God’s "sweet providence" would never allow the one who could not teach error to fall into error, and that this was guaranteed by the promise Ego autem rogavi pro te . . . (Luke 22: 32). But, like Bellarmine, Suarez was willing to consider the possibility of an heretical pope as an hypothesis, particularly in view of the fact, he claimed, that several "general councils had admitted the hypothesis in question".(5) Saint Alphonsus Liguori (1696-1787) did not believe that God would ever permit a Roman Pontiff to become a public or an occult heretic, even as a private person: "We ought rightly to presume as Cardinal Bellarmine declares, that God will never let it happen that a Roman Pontiff, even as a private person, becomes a public heretic or an occult heretic."(6)

If, per impossible, a pope became a formal heretic through pertinaciously denying a de fide doctrine, how would the faithful know that he had forfeited his office as he had ceased to be a Catholic? It must be remembered that no one in the Church, including a General Council, has the authority to judge the Popes. Reputable authorities teach that if a pope did pertinaciously deny a truth which must be believed by divine and Catholic faith, after this had been brought to his attention by responsible members of the hierarchy (just as St. Paul reproved St. Peter to his face), a General Council could announce to the Church that the Pope, as a notorious heretic, had ceased to be a Catholic and hence had ceased to be Pope. It is important to note that the Council would neither be judging nor deposing the Pope, since it would not possess the authority for such an act. It would simply be making a declaratory sentence, i.e. declaring to the Church what had already become manifest from the Pope’s own actions. This is the view taken in the classic manual on Canon Law by Father F.X. Wernz, Rector of the Gregorian University and Jesuit General from 1906 to 1914. This work was revised by Father P. Vidal and was last republished in 1952. It states clearly that an heretical Pope is not deposed in virtue of the sentence of the Council, but "the General Council declares the fact of the crime by which the heretical pope has separated himself from the Church and deprived himself of his dignity."(7) Other authorities believe that such a declaration could come from the College of Cardinals or from a representative group of bishop, while others maintain that such a declaration would not be necessary. What all those who accept the hypothesis of an heretical pope are agreed upon is that for such a pope to forfeit the papacy his heresy would have to be "manifest", as Saint Robert Bellarmine expressed it, that is notorious and public (notorium et palam divulgata).(8) A notorious offence can be defined as one for which the evidence is so certain that it can in no way be either hidden or excused.(9) A pope who, while not being guilty of formal heresy in the strict sense, has allowed heresy to undermine the Church through compromise, weakness, ambiguous or even gravely imprudent teaching remains Pope, but can be judged by his successors, and condemned as was the case with Honorius I.