Objection 1. It would seem that this sacrament was unsuitably instituted in the New Law. Because those things which belong to the natural law need not to be instituted. Now it belongs to the natural law that one should repent of the evil one has done: for it is impossible to love good without grieving for its contrary. Therefore Penance was unsuitably instituted in the New Law.
Objection 2. Further, that which existed in the Old Law had not to be instituted in the New. Now there was Penance in the old Law wherefore the Lord complains (Jeremiah 8:6) saying: "There is none that doth penance for his sin, saying: What have I done?" Therefore Penance should not have been instituted in the New Law.
Objection 3. Further, Penance comes after Baptism, since it is a second plank, as stated above (Article 6). Now it seems that our Lord instituted Penance before Baptism, because we read that at the beginning of His preaching He said (Matthew 4:17): "Do penance, for the kingdom of heaven is at hand." Therefore this sacrament was not suitably instituted in the New Law.
Objection 4. Further, the sacraments of the New Law were instituted by Christ, by Whose power they work, as stated above (62, 5; 64, 1). But Christ does not seem to have instituted this sacrament, since He made no use of it, as of the other sacraments which He instituted. Therefore this sacrament was unsuitably instituted in the New Law.
On the contrary, our Lord said (Luke 24:46-47): "It behooved Christ to suffer, and to rise again from the dead the third day: and that penance and remission of sins should be preached in His name unto all nations."
I answer that, As stated above (1, ad 1, ad 2), in this sacrament the acts of the penitent are as matter, while the part taken by the priest, who works as Christ's minister, is the formal and completive element of the sacrament. Now in the other sacraments the matter pre-exists, being provided by nature, as water, or by art, as bread: but that such and such a matter be employed for a sacrament requires to be decided by the institution; while the sacrament derives its form and power entirely from the institution of Christ, from Whose Passion the power of the sacraments proceeds.
Accordingly the matter of this sacrament pre-exists, being provided by nature; since it is by a natural principle of reason that man is moved to repent of the evil he has done: yet it is due to Divine institution that man does penance in this or that way. Wherefore at the outset of His preaching, our Lord admonished men, not only to repent, but also to "do penance," thus pointing to the particular manner of actions required for this sacrament. As to the part to be taken by the ministers, this was fixed by our Lord when He said to Peter (Matthew 16:19): "To thee will I give the keys of the kingdom of heaven," etc.; but it was after His resurrection that He made known the efficacy of this sacrament and the source of its power, when He said (Luke 24:47) that "penance and remission of sins should be preached in His name unto all nations," after speaking of His Passion and resurrection. Because it is from the power of the name of Jesus Christ suffering and rising again that this sacrament is efficacious unto the remission of sins.
It is therefore evident that this sacrament was suitably instituted in the New Law.
Reply to Objection 1. It is a natural law that one should repent of the evil one has done, by grieving for having done it, and by seeking a remedy for one's grief in some way or other, and also that one should show some signs of grief, even as the Ninevites did, as we read in Jonah 3. And yet even in their case there was also something of faith which they had received through Jonas' preaching, inasmuch as they did these things in the hope that they would receive pardon from God, according as we read (Jonah 3:9): "Who can tell if God will turn and forgive, and will turn away from His fierce anger, and we shall not perish?" But just as other matters which are of the natural law were fixed in detail by the institution of the Divine law, as we have stated in I-II, 91, 4; I-II, 95, 2; I-II, 99, so was it with Penance.
Reply to Objection 2. Things which are of the natural law were determined in various ways in the old and in the New Law, in keeping with the imperfection of the old, and the perfection of the New. Wherefore Penance was fixed in a certain way in the Old Law--with regard to sorrow, that it should be in the heart rather than in external signs, according to Joel 2:13: "Rend your hearts and not your garments"; and with regard to seeking a remedy for sorrow, that they should in some way confess their sins, at least in general, to God's ministers. Wherefore the Lord said (Leviticus 5:17-18): "If anyone sin through ignorance... he shall offer of the flocks a ram without blemish to the priest, according to the measure and estimation of the sin, and the priest shall pray for him, because he did it ignorantly, and it shall be forgiven him"; since by the very fact of making an offering for his sin, a man, in a fashion, confessed his sin to the priest. And accordingly it is written (Proverbs 28:13): "He that hideth his sins, shall not prosper: but he that shall confess, and forsake them, shall obtain mercy." Not yet, however, was the power of the keys instituted, which is derived from Christ's Passion, and consequently it was not yet ordained that a man should grieve for his sin, with the purpose of submitting himself by confession and satisfaction to the keys of the Church, in the hope of receiving forgiveness through the power of Christ's Passion.
Reply to Objection 3. If we note carefully what our Lord said about the necessity of Baptism (John 3:3, sqq.), we shall see that this was said before His words about the necessity of Penance (Matthew 4:17); because He spoke to Nicodemus about Baptism before the imprisonment of John, of whom it is related afterwards (John 3:23-24) that he baptized, whereas His words about Penance were said after John was cast into prison.
If, however, He had admonished men to do penance before admonishing them to be baptized, this would be because also before Baptism some kind of penance is required, according to the words of Peter (Acts 2:38): "Do penance, and be baptized, every one of you."
Reply to Objection 4. Christ did not use the Baptism which He instituted, but was baptized with the baptism of John, as stated above (39, 1,2). Nor did He use it actively by administering it Himself, because He "did not baptize" as a rule, "but His disciples" did, as related in John 4:2, although it is to be believed that He baptized His disciples, as Augustine asserts (Ep. cclxv, ad Seleuc.). But with regard to His institution of this sacrament it was nowise fitting that He should use it, neither by repenting Himself, in Whom there was no sin, nor by administering the sacrament to others, since, in order to show His mercy and power, He was wont to confer the effect of this sacrament without the sacrament itself, as stated above (5, ad 3). On the other hand, He both received and gave to others the sacrament of the Eucharist, both in order to commend the excellence of that sacrament, and because that sacrament is a memorial of His Passion, in which Christ is both priest and victim.
Source: Summa Theologica (New Advent)