I answer that, It is impossible for a mortal actual sin to be pardoned without penance, if we speak of penance as a virtue. For, as sin is an offense against God, He pardons sin in the same way as he pardons an offense committed against Him. Now an offense is directly opposed to grace, since one man is said to be offended with another, because he excludes him from his grace. Now, as stated in the Second Part (I-II, Q. 110, A. 1), the difference between the grace of God and the grace of man, is that the latter does not cause, but presupposes true or apparent goodness in him who is graced, whereas the grace of God causes goodness in the man who is graced, because the good-will of God, which is denoted by the word "grace," is the cause of all created good. Hence it is possible for a man to pardon an offense, for which he is offended with someone, without any change in the latter's will; but it is impossible that God pardon a man for an offense, without his will being changed. Now the offense of mortal sin is due to man's will being turned away from God, through being turned to some mutable good. Consequently, for the pardon of this offense against God, it is necessary for man's will to be so changed as to turn to God and to renounce having turned to something else in the aforesaid manner, together with a purpose of amendment; all of which belongs to the nature of penance as a virtue. Therefore it is impossible for a sin to be pardoned anyone without penance as a virtue.
But the sacrament of Penance, as stated above (Q. 88, A. 3), is perfected by the priestly office of binding and loosing, without which God can forgive sins, even as Christ pardoned the adulterous woman, as related in John 8, and the woman that was a sinner, as related in Luke vii, whose sins, however, He did not forgive without the virtue of penance: for as Gregory states (Hom. xxxiii in Evang.), "He drew inwardly by grace," i.e. by penance, "her whom He received outwardly by His mercy."
Reply Obj. 1: In children there is none but original sin, which consists, not in an actual disorder of the will, but in a habitual disorder of nature, as explained in the Second Part (I-II, Q. 82, A. 1), and so in them the forgiveness of sin is accompanied by a habitual change resulting from the infusion of grace and virtues, but not by an actual change. On the other hand, in the case of an adult, in whom there are actual sins, which consist in an actual disorder of the will, there is no remission of sins, even in Baptism, without an actual change of the will, which is the effect of Penance.
Reply Obj. 2: This argument takes Penance as a sacrament.
Reply Obj. 3: God's mercy is more powerful than man's, in that it moves man's will to repent, which man's mercy cannot do. _______________________
THIRD ARTICLE [III, Q. 86, Art. 3]
Whether by Penance One Sin Can Be Pardoned Without Another?
Objection 1: It would seem that by Penance one sin can be pardoned without another. For it is written (Amos 4:7): "I caused it to rain upon one city, and caused it not to rain upon another city; one piece was rained upon: and the piece whereupon I rained not, withered." These words are expounded by Gregory, who says (Hom. x super Ezech.): "When a man who hates his neighbor, breaks himself of other vices, rain falls on one part of the city, leaving the other part withered, for there are some men who, when they prune some vices, become much more rooted in others." Therefore one sin can be forgiven by Penance, without another.
Obj. 2: Further, Ambrose in commenting on Ps. 118, "Blessed are the undefiled in the way," after expounding verse 136 ("My eyes have sent forth springs of water"), says that "the first consolation is that God is mindful to have mercy; and the second, that He punishes, for although faith be wanting, punishment makes satisfaction and raises us up." Therefore a man can be raised up from one sin, while the sin of unbelief remains.
Obj. 3: Further, when several things are not necessarily together, one can be removed without the other. Now it was stated in the Second Part (I-II, Q. 73, A. 1) that sins are not connected together, so that one sin can be without another. Therefore also one sin can be taken away by Penance without another being taken away.