Objection 1. It would seem that sin can be pardoned without Penance. For the power of God is no less with regard to adults than with regard to children. But He pardons the sins of children without Penance. Therefore He also pardons adults without penance.
Objection 2. Further, God did not bind His power to the sacraments. But Penance is a sacrament. Therefore by God's power sin can be pardoned without Penance.
Objection 3. Further, God's mercy is greater than man's. Now man sometimes forgives another for offending him, without his repenting: wherefore our Lord commanded us (Matthew 5:44): "Love your enemies, do good to them that hate you." Much more, therefore, does God pardon men for offending him, without their repenting.
On the contrary, The Lord said (Jeremiah 18:8): "If that nation... shall repent of their evil" which they have done, "I also will repent of the evil that I have thought to do them," so that, on the other hand, if man "do not penance," it seems that God will not pardon him his sin.
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 I-II, 110, 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 (Question 88, Article 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 to Objection 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 I-II, 82, 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 to Objection 2. This argument takes Penance as a sacrament.
Reply to Objection 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.
Source: Summa Theologica (New Advent)