Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We must now consider absolution from excommunication: under which head there are three points of inquiry:
(1) Whether any priest can absolve his subject from excommunication?
(2) Whether a man can be absolved from excommunication against his will?
(3) Whether a man can be absolved from one excommunication without being absolved from another?
Objection 1: It would seem that any priest can absolve his subject from excommunication. For the chains of sin are stronger than those of excommunication. But any priest can absolve his subject from sin. Therefore much more can he absolve him from excommunication.
Objection 2: Further, if the cause is removed the effect is removed. But the cause of excommunication is a mortal sin. Therefore since any priest can absolve (his subject) from that mortal sin, he is able likewise to absolve him from the excommunication.
On the contrary, It belongs to the same power to excommunicate as to absolve from excommunication. But priests of inferior degree cannot excommunicate their subjects. Neither, therefore, can they absolve them.
I answer that, Anyone can absolve from minor excommunication who can absolve from the sin of participation in the sin of another. But in the case of a major excommunication, this is pronounced either by a judge, and then he who pronounced sentence or his superior can absolve---or it is pronounced by law, and then the bishop or even a priest can absolve except in the six cases which the Pope, who is the maker of laws, reserves to himself: the first is the case of a man who lays hands on a cleric or a religious; the second is of one who breaks into a church and is denounced for so doing; the third is of the man who sets fire to a church and is denounced for the deed; the fourth is of one who knowingly communicates in the Divine worship with those whom the Pope has excommunicated by name; the fifth is the case of one who tampers with the letters of the Holy See; the sixth is the case of one who communicates in a crime of one who is excommunicated. For he should not be absolved except by the person who excommunicated him, even though he be not subject to him, unless, by reason of the difficulty of appearing before him, he be absolved by the bishop or by his own priest, after binding himself by oath to submit to the command of the judge who pronounced the excommunication on him.
There are however eight exceptions to the first case: (1) In the hour of death, when a person can be absolved by any priest from any excommunication; (2) if the striker be the doorkeeper of a man in authority, and the blow be given neither through hatred nor of set purpose; (3) if the striker be a woman; (4) if the striker be a servant, whose master is not at fault and would suffer from his absence; (5) if a religious strike a religious, unless he strike him very grievously; (6) if the striker be a poor man; (7) if he be a minor, an old man, or an invalid; (8) if there be a deadly feud between them.
There are, besides, seven cases in which the person who strikes a cleric does not incur excommunication: (1) if he do it for the sake of discipline, as a teacher or a superior; (2) if it be done for fun; (3) if the striker find the cleric behaving with impropriety towards his wife his mother, his sister or his daughter; (4) if he return blow for blow at once; (5) if the striker be not aware that he is striking a cleric; (6) if the latter be guilty of apostasy after the triple admonition; (7) if the cleric exercise an act which is altogether contrary to the clerical life, e.g. if he become a soldier, or if he be guilty of bigamy [*Namely, that which is known by canonists as "similar bigamy"].
Reply to Objection 1: Although the chains of sin are in themselves greater than those of excommunication, yet in a certain respect the chains of excommunication are greater, inasmuch as they bind a man not only in the sight of God, but also in the eye of the Church. Hence absolution from excommunication requires jurisdiction in the external forum, whereas absolution from sin does not. Nor is there need of giving one's word by oath, as in the case of absolution from excommunication, because, as the Apostle declares (Heb. 6:16), controversies between men are decided by oath.
Reply to Objection 2: As an excommunicated person has no share in the sacraments of the Church, a priest cannot absolve him from his guilt, unless he be first absolved from excommunication.
Objection 1: It would seem that no man can be absolved against his will. For spiritual things are not conferred on anyone against his will. Now absolution from excommunication is a spiritual favor. Therefore it cannot be granted to a man against his will.
Objection 2: Further, the cause of excommunication is contumacy. But when, through contempt of the excommunication, a man is unwilling to be absolved, he shows a high degree of contumacy. Therefore he cannot be absolved.
On the contrary, Excommunication can be pronounced on a man against his will. Now things that happen to a man against his will, can be removed from him against his will, as in the case of the goods of fortune. Therefore excommunication can be removed from a man against his will.
I answer that, Evil of fault and evil of punishment differ in this, that the origin of fault is within us, since all sin is voluntary, whereas the origin of punishment is sometimes without, since punishment does not need to be voluntary, in fact the nature of punishment is rather to be against the will. Wherefore, just as a man commits no sin except willingly, so no sin is forgiven him against his will. On the other hand just as a person can be excommunicated against his will, so can he be absolved therefrom.
Reply to Objection 1: The assertion is true of those spiritual goods which depend on our will, such as the virtues, which we cannot lose unwillingly; for knowledge, although a spiritual good, can be lost by a man against his will through sickness. Hence the argument is not to the point.
Reply to Objection 2: It is possible for excommunication to be removed from a man even though he be contumacious, if it seem to be for the good of the man for whom the excommunication was intended as a medicine.
Objection 1: It would seem that a man cannot be absolved from one excommunication without being absolved from all. For an effect should be proportionate to its cause. Now the cause of excommunication is a sin. Since then a man cannot be absolved from one sin without being absolved from all, neither can this happen as regards excommunication.
Objection 2: Further, absolution from excommunication is pronounced in the Church. But a man who is under the ban of one excommunication is outside the Church. Therefore so long as one remains, a man cannot be loosed from another.
On the contrary, Excommunication is a punishment. Now a man can be loosed from one punishment, while another remains. Therefore a man can be loosed from one excommunication and yet remain under another.
I answer that, Excommunications are not connected together in any way, and so it is possible for a man to be absolved from one, and yet remain under another.
It must be observed however that sometimes a man lies under several excommunications pronounced by one judge; and then, when he is absolved from one, he is understood to be absolved from all, unless the contrary be expressed, or unless he ask to be absolved from excommunication on one count only, whereas he was excommunicated under several. On the other hand sometimes a man lies under several sentences of excommunication pronounced by several judges; and then, when absolved from one excommunication, he is not therefore absolved from the others, unless at his prayer they all confirm his absolution, or unless they all depute one to absolve him.
Reply to Objection 1: All sins are connected together in aversion from God, which is incompatible with the forgiveness of sin: wherefore one sin cannot be forgiven without another. But excommunications have no such connection. Nor again is absolution from excommunication hindered by contrariety of the will, as stated above (A[2]). Hence the argument does not prove.
Reply to Objection 2: Just as such a man was for several reasons outside the Church so is it possible for his separation to be removed on one count and to remain on another.