Objection 1. It would seem that the power of the keys extends to the remission of guilt. For it was said to the disciples (John 20:23): "Whose sins you shall forgive, they are forgiven them." Now this was not said in reference to the declaration only, as the Master states (Sent. iv, D, 18), for in that case the priest of the New Testament would have no more power than the priest of the Old Testament. Therefore he exercises a power over the remission of the guilt.
Objection 2. Further, in Penance grace is given for the remission of sin. Now the priest is the dispenser of this sacrament by virtue of the keys. Therefore, since grace is opposed to sin, not on the part of the punishment, but on the part of the guilt, it seems that the priest operates unto the remission of sin by virtue of the keys.
Objection 3. Further, the priest receives more power by his consecration than the baptismal water by its sanctification. Now the baptismal water receives the power "to touch the body and cleanse the heart," as Augustine says (Tract. lxxx in Joan.). Much more, therefore, does the priest, in his consecration, receive the power to cleanse the heart from the stain of sin.
On the contrary, The Master stated above (Sent. iv, D, 18) that God has not bestowed on the minister the power to co-operate with Him in the inward cleansing. Now if he remitted sins as to the guilt, he would co-operate with God in the inward cleansing. Therefore the power of the keys does not extend to the remission of guilt.
Further, sin is not remitted save by the Holy Ghost. But no man has the power to give the Holy Ghost, as the Master said above (Sent. i, D, 14). Neither therefore can he remit sins as to their guilt.
I answer that, According to Hugh (De Sacram. ii), "the sacraments, by virtue of their sanctification, contain an invisible grace." Now this sanctification is sometimes essential to the sacrament both as regards the matter and as regards the minister, as may be seen in Confirmation, and then the sacramental virtue is in both together. Sometimes, however, the essence of the sacrament requires only sanctification of the matter, as in Baptism, which has no fixed minister on whom it depends necessarily, and then the whole virtue of the sacrament is in the matter. Again, sometimes the essence of the sacrament requires the consecration or sanctification of the minister without any sanctification of the matter, and then the entire sacramental virtue is in the minister, as in Penance. Hence the power of the keys which is in the priest, stands in the same relation to the effect of Penance, as the virtue in the baptismal water does to the effect of Baptism. Now Baptism and the sacrament of Penance agree somewhat in their effect, since each is directly ordained against guilt, which is not the case in the other sacraments: yet they differ in this, that the sacrament of Penance, since the acts of the recipient are as its matter, cannot be given save to adults, who need to be disposed for the reception of the sacramental effect; whereas Baptism is given, sometimes to adults, sometimes to children and others who lack the use of reason, so that by Baptism children receive grace and remission of sin without any previous disposition, while adults do not, for they require to be disposed by the removal of insincerity. This disposition sometimes precedes their Baptism by priority of time, being sufficient for the reception of grace, before they are actually baptized, but not before they have come to the knowledge of the truth and have conceived the desire for Baptism. At other times this disposition does not precede the reception of Baptism by a priority of time, but is simultaneous with it, and then the grace of the remission of guilt is bestowed through the reception of Baptism. On the other hand, grace is never given through the sacrament of Penance unless the recipient be disposed either simultaneously or before. Hence the power of the keys operates unto the remission of guilt, either through being desired or through being actually exercised, even as the waters of Baptism. But just as Baptism acts, not as a principal agent but as an instrument, and does not go so far as to cause the reception itself of grace, even instrumentally [See note at beginning of this article], but merely disposes the recipient to the grace whereby his guilt is remitted, so is it with the power of the keys. Wherefore God alone directly remits guilt, and Baptism acts through His power instrumentally, as an inanimate instrument, and the priest as an animate instrument, such as a servant is, according to the Philosopher (Ethic. viii, 11): and consequently the priest acts as a minister. Hence it is clear that the power of the keys is ordained, in a manner, to the remission of guilt, not as causing that remission, but as disposing thereto. Consequently if a man, before receiving absolution, were not perfectly disposed for the reception of grace, he would receive grace at the very time of sacramental confession and absolution, provided he offered no obstacle. For if the key were in no way ordained to the remission of guilt, but only to the remission of punishment, as some hold, it would not be necessary to have a desire of receiving the effect of the keys in order to have one's sins forgiven, just as it is not necessary to have a desire of receiving the other sacraments which are ordained, not to the remission of guilt, but against punishment. But this enables us to see that it is not ordained unto the remission of guilt, because the use of the keys, in order to be effective, always requires a disposition on the part of the recipient of the sacrament. And the same would apply to Baptism, were it never given save to adults.
Reply to Objection 1. As the Master says in the text (Sent. iv, D, 18), the power of forgiving sins was entrusted to priests, not that they may forgive them, by their own power, for this belongs to God, but that, as ministers, they may declare [See note at the beginning of this article] the operation of God Who forgives. Now this happens in three ways. First, by a declaration, not of present, but of future forgiveness, without co-operating therein in any way: and thus the sacraments of the Old Law signified the Divine operation, so that the priest of the Old Law did but declare and did not operate the forgiveness of sins. Secondly, by a declaration of present forgiveness without co-operating in it at all: and thus some say that the sacraments of the New Law signify the bestowal of grace, which God gives when the sacraments are conferred, without the sacraments containing any power productive of grace, according to which opinion, even the power of the keys would merely declare the Divine operation that has its effect in the remission of guilt when the sacrament is conferred. Thirdly, by signifying the Divine operation causing then and there the remission of guilt, and by co-operating towards this effect dispositively and instrumentally: and then, according to another and more common opinion, the sacraments of the New Law declare the cleansing effected by God. In this way also the priest of the New Testament declares the recipient to be absolved from guilt, because in speaking of the sacraments, what is ascribed to the power of the ministers must be consistent with the sacrament. Nor is it unreasonable that the keys of the Church should dispose the penitent to the remission of his guilt, from the fact that the guilt is already remitted, even as neither is it unreasonable that Baptism, considered in itself, causes a disposition in one who is already sanctified.
Reply to Objection 2. Neither the sacrament of Penance, nor the sacrament of Baptism, by its operation, causes grace, or the remission of guilt, directly, but only dispositively [St. Thomas here follows the opinion of Peter Lombard, and replies in the negative. Later in life he altered his opinion. Cf. III, 62, 1; III, 64, 1; III, 86, 6]. Hence the Reply to the Third Objection is evident. The other arguments show that the power of the keys does not effect the remission of guilt directly, and this is to be granted.
Source: Summa Theologica (New Advent)