Pope Paul IV’s bull Cum Ex Apostolatus (15th February 1559, Bullarium Romanum vol. iv. sect. i, pp. 354-357) “…if ever the cardinals should elect as pope someone who was guilty of prior heresy, the election would be simply null and the faithful would have the entire right to withdraw from obedience to the person elected, as he would not be their head.”
This bull, and in the mind of Pope Paul IV, aimed particularly at excluding the possibility that after his death the conclave would elect a heretic never condemned by the Church.
St. Robert Bellarmine, Cardinal and Doctor of the Church, De Romano Pontifice, II, 30: “A pope who is a manifest heretic automatically (per se) ceases to be pope and head, just as he ceases automatically to be a Christian and a member of the Church. Wherefore, he can be judged and punished by the Church. This is the teaching of all the ancient Fathers who teach that manifest heretics immediately lose all jurisdiction.”
St. Robert Bellarmine, De Romano Pontifice, II, 30:
“This principle is most certain. The non-Christian cannot in any way be Pope, as Cajetan himself admits (ib. c. 26). The reason for this is that he cannot be head of what he is not a member; now he who is not a Christian is not a member of the Church, and a manifest heretic is not a Christian, as is clearly taught by St. Cyprian (lib. 4, epist. 2), St. Athanasius (Scr. 2 cont. Arian.),
St. Augustine (lib. De great. Christ. Cap. 20), St. Jerome (contra Lucifer.) and others; therefore, the manifest heretic cannot be Pope.”
St. Francis De Sales (17th century), Doctor of the Church, The Catholic Controversy, pp. 305-306: “Now when he [the Pope] is explicitly a heretic, he falls ipso facto from his dignity and out of the Church…”
St. Antoninus (1459): “In the case in which the pope would become a heretic, he would find himself, by that fact alone and without any other sentence, separated from the Church. A head separated from a body cannot, as long as it remains separated, be head of the same body from which it was cut off. A pope who would be separated from the Church by heresy, therefore, would by that very fact itself cease to be head of the Church. He could not be a heretic and remain pope, because, since he is outside of the Church, he cannot possess the keys of the Church.” (Summa Theologica, cited in Actes de Vatican I. V. Frond pub.)
The Catholic Encyclopedia, “Heresy,” 1914, Vol. 7, p. 261: “The pope himself, if notoriously guilty of heresy, would cease to be pope because he would cease to be a member of the Church.”
Pope Pius XII, Mystici Corporis Christi (# 23), June 29, 1943:
“For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy.”
A heretic cannot be the pope (33) We can see that it’s the teaching of the Catholic Church that a man is severed from the Church by heresy, schism or apostasy.
Pope Leo XIII, Satis Cognitum (# 9), June 29, 1896:
“The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium.”4
Pope Leo XIII, Satis Cognitum (# 9):
“No one who merely disbelieves in all (these heresies) can for that reason regard himself as a Catholic or call himself one. For there may be or arise some other heresies, which are not set out in this work of ours, and, if any one holds to a single one of these he is not a Catholic.”5
Pope Innocent III, Eius exemplo, Dec. 18, 1208:
“By the heart we believe and by the mouth we confess the one Church, not of heretics, but the Holy Roman, Catholic, and Apostolic Church outside of which we believe that no one is saved.”
Thus, it’s not merely the opinion of certain saints and doctors of the Church that a heretic would cease to be pope; it’s a fact inextricably bound up with a dogmatic teaching. A truth inextricably bound up with a dogma is called a dogmatic fact. It is, therefore, a dogmatic fact that a heretic cannot be the pope. A heretic cannot be the pope, since one who is outside cannot head that of which he is not even a member.
Pope Leo XIII, Satis Cognitum (#15), June 29, 1896:
“No one, therefore, unless in communion with Peter can share in his authority, since it is absurd to imagine that he who is outside can command in the Church.”
Canonicum 1:415 ” Those capable of being validly elected are all who are not prohibited by divine law or by an invalidating ecclesiastical law… those who are barred as incapable of being validly elected are all women, children, who have not reached the age of reason; also those afflicted with habitual insanity, the unbaptized, Heretics, Schismatics.