Reply Obj. 1: This argument considers the proximate matter of a sacrament.
Reply Obj. 2: The old life that was subject to death is the object of Penance, not as regards the punishment, but as regards the guilt connected with it.
Reply Obj. 3: Penance regards every kind of sin in a way, but not each in the same way. Because Penance regards actual mortal sin properly and chiefly; properly, since, properly speaking, we are said to repent of what we have done of our own will; chiefly, since this sacrament was instituted chiefly for the blotting out of mortal sin. Penance regards venial sins, properly speaking indeed, in so far as they are committed of our own will, but this was not the chief purpose of its institution. But as to original sin, Penance regards it neither chiefly, since Baptism, and not Penance, is ordained against original sin, nor properly, because original sin is not done of our own will, except in so far as Adam's will is looked upon as ours, in which sense the Apostle says (Rom. 5:12): "In whom all have sinned." Nevertheless, Penance may be said to regard original sin, if we take it in a wide sense for any detestation of something past: in which sense Augustine uses the term in his book De Poenitentia (Serm. cccli). _______________________
THIRD ARTICLE [III, Q. 84, Art. 3]
Whether the Form of This Sacrament Is: "I Absolve Thee"?
Objection 1: It would seem that the form of this sacrament is not: "I absolve thee." Because the forms of the sacraments are received from Christ's institution and the Church's custom. But we do not read that Christ instituted this form. Nor is it in common use; in fact in certain absolutions which are given publicly in church (e.g. at Prime and Compline and on Maundy Thursday), absolution is given not in the indicative form by saying: "I absolve thee," but in the deprecatory form, by saying: "May Almighty God have mercy on you," or: "May Almighty God grant you absolution and forgiveness." Therefore the form of this sacrament is not: "I absolve thee."
Obj. 2: Further, Pope Leo says (Ep. cviii) that God's forgiveness cannot be obtained without the priestly supplications: and he is speaking there of God's forgiveness granted to the penitent. Therefore the form of this sacrament should be deprecatory.
Obj. 3: Further, to absolve from sin is the same as to remit sin. But God alone remits sin, for He alone cleanses man inwardly from sin, as Augustine says (Contra Donatist. v, 21). Therefore it seems that God alone absolves from sin. Therefore the priest should say not: "I absolve thee," as neither does he say: "I remit thy sins."
Obj. 4: Further, just as our Lord gave His disciples the power to absolve from sins, so also did He give them the power "to heal infirmities," "to cast out devils," and "to cure diseases" (Matt. 10:1; Luke 9:1). Now the apostles, in healing the sick, did not use the words: "I heal thee," but: "The Lord Jesus Christ heal [Vulg.: 'heals'] thee," as Peter said to the palsied man (Acts 9:34). Therefore since priests have the power which Christ gave His apostles, it seems that they should not use the form: "I absolve thee," but: "May Christ absolve thee."
Obj. 5: Further, some explain this form by stating that when they say: "I absolve thee," they mean "I declare you to be absolved." But neither can this be done by a priest unless it be revealed to him by God, wherefore, as we read in Matt. 16:19 before it was said to Peter: "Whatsoever thou shalt bind upon earth," etc., it was said to him (Matt. 16:17): "Blessed art thou Simon Bar-Jona: because flesh and blood have not revealed it to thee, but My Father Who is in heaven." Therefore it seems presumptuous for a priest, who has received no revelation on the matter, to say: "I absolve thee," even if this be explained to mean: "I declare thee absolved."