Book I, CHAPTER IX
On the utility or even the necessity of celebrating Councils
Therefore, with all of this noted, we must explain in what things legitimate Councils consist, and these can be reduced to four: 1) the end; 2) efficiency; 3) matter and; 4) the form of Councils. Now let us begin with the end, which is the first of these reasons. It will be the first reason that must be briefly explained on account of which Councils are usually celebrated; then from those it will be determined whether a gathering of Councils is necessary or merely useful. Moreover, the particular reasons on account of which Councils are celebrated are usually numbered as six.
a) The first reason is a new heresy, i.e. something that had never been judged before, which is the very reason the first seven Councils were convened. The Church always so dealt with the danger of new heresies that she did not think it could be resisted otherwise than if all or certainly a great many leaders of the Churches, once their strength was joined as if it were made into a column of soldiers, would rush upon the enemies of the faith.
b) The second reason is schism among Roman Pontiffs; for a Council in the time of Pope Cornelius was celebrated for this very reason. Likewise, another in the time of Pope Damasus and again in the times of Symmachus, Innocent II and Alexander III, as well as Pisa and Constance in the times of Gregory XII and Benedict XIII, for there is no more powerful remedy than a Council as has so often been proved.
c) The third is resistance to a common enemy of the whole Church; in this manner Councils were convened by Urban II, Calixtus II, Eugene III, and other Popes, for war against the Saracens. Likewise, to depose an emperor, Gregory III celebrated Councils against Leo III the Iconoclast, as did Gregory VII against Henry IV, and Innocent IV against Frederick II.
d) The fourth reason is suspicion of heresy in the Roman Pontiff, if perhaps it might happen, or if he were an incorrigible tyrant; for then a general Council ought to be gathered either to depose the Pope if he should be found to be a heretic, or certainly to admonish him if he seemed incorrigible in morals. As it is related in the 8th Council, act. ult. can. 21, general Councils ought to impose judgment on controversies arising in regard to the Roman Pontiff—albeit not rashly. For this reason we read that the Council of Sinvessano in the case of St. Marcellinus, as well as Roman Councils in the cases of Pope Damasus, Sixtus III, and Symmachus, as well as Leo III and IV, none of whom were condemned by a Council; Marcellinus enjoined penance upon himself in the presence of the Council, and the rest purged themselves (See Platina and the volumes of Councils).
e) The fifth reason is doubt about the election of a Roman Pontiff. For if the cardinals could not or would not create a Pope, or certainly if they all died at the same time, or a true doubt should arise for another reason to whom an election of this sort would pertain, would look to a general Council to discern in regard to the election of a future Pope, although it does not seem to be realistic to expect this would ever happen.
f) The sixth reason is the general reformation of abuses and vices which crept into the Church; for even if the Pope alone can prescribe laws for the whole Church, nevertheless, it is by far more agreeable for matters to be done with the approval of a general Council when the Pope prescribes laws of this sort. Hence, we see nearly all general Councils published canons on reformation (See Juan Torquemada, lib. 3, cap. 9 &10).