Objection 1. It would seem that a priest cannot always absolve his subject. For, as Augustine says (De vera et false Poenitentia [Work of an unknown author]), "no man should exercise the priestly office, unless he be free from those things which he condemns in others." But a priest might happen to share in a sin committed by his subject, e.g. by knowledge of a woman who is his subject. Therefore it seems that he cannot always use the power of the keys on his subjects.
Objection 2. Further, by the power of the keys a man is healed of all his shortcomings. Now it happens sometimes that a sin has attached to it a defect of irregularity or a sentence of excommunication, from which a simple priest cannot absolve. Therefore it seems that he cannot use the power of the keys on such as are shackled by these things in the above manner.
Objection 3. Further, the judgment and power of our priesthood was foreshadowed by the judgment of the ancient priesthood. Now according to the Law, the lesser judges were not competent to decide all cases, and had recourse to the higher judges, according to Exodus 24:14: "If any question shall arise" among you, "you shall refer it to them." It seems, therefore, that a priest cannot absolve his subject from graver sins, but should refer him to his superior.
On the contrary, Whoever has charge of the principal has charge of the accessory. Now priests are charged with the dispensation of the Eucharist to their subjects, to which sacrament the absolution of sins is subordinate [Cf. 17, 2, ad 1]. Therefore, as far as the power of the keys is concerned, a priest can absolve his subject from any sins whatever.
Further, grace, however small, removes all sin. But a priest dispenses sacraments whereby grace is given. Therefore, as far as the power of the keys is concerned, he can absolve from all sins.
I answer that, The power of order, considered in itself, extends to the remission of all sins. But since, as stated above, the use of this power requires jurisdiction which inferiors derive from their superiors, it follows that the superior can reserve certain matters to himself, the judgment of which he does not commit to his inferior; otherwise any simple priest who has jurisdiction can absolve from any sin. Now there are five cases in which a simple priest must refer his penitent to his superior. The first is when a public penance has to be imposed, because in that case the bishop is the proper minister of the sacrament. The second is the case of those who are excommunicated when the inferior priest cannot absolve a penitent through the latter being excommunicated by his superior. The third case is when he finds that an irregularity has been contracted, for the dispensation of which he has to have recourse to his superior. The fourth is the case of arson. The fifth is when it is the custom in a diocese for the more heinous crimes to be reserved to the bishop, in order to inspire fear, because custom in these cases either gives the power or takes it away.
Reply to Objection 1. In this case the priest should not hear the confession of his accomplice, with regard to that particular sin, but must refer her to another: nor should she confess to him but should ask permission to go to another, or should have recourse to his superior if he refused, both on account of the danger, and for the sake of less shame. If, however, he were to absolve her it would be valid*: because when Augustine says that they should not be guilty of the same sin, he is speaking of what is congruous, not of what is essential to the sacrament. [Benedict XIV declared the absolution of an accomplice "in materia turpi" to be invalid.]
Reply to Objection 2. Penance delivers man from all defects of guilt, but not from all defects of punishment, since even after doing penance for murder, a man remains irregular. Hence a priest can absolve from a crime, but for the remission of the punishment he must refer the penitent to the superior, except in the case of excommunication, absolution from which should precede absolution from sin, for as long as a man is excommunicated, he cannot receive any sacrament of the Church.
Reply to Objection 3. This objection considers those cases in which superiors reserve the power of jurisdiction to themselves.
Source: Summa Theologica (New Advent)