Reply Obj. 2: Penance cannot be called the foundation of the spiritual edifice simply, i.e. in the first building thereof; but it is the foundation in the second building which is accomplished by destroying sin, because man, on his return to God, needs Penance first. However, the Apostle is speaking there of the foundation of spiritual doctrine. Moreover, the penance which precedes Baptism is not the sacrament of Penance.
Reply Obj. 3: The three sacraments which precede Penance refer to the ship in its integrity, i.e. to man's state of integrity, with regard to which Penance is called a second plank. _______________________
SEVENTH ARTICLE [III, Q. 84, Art. 7]
Whether This Sacrament Was Suitably Instituted in the New Law?
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.
Obj. 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 (Jer. 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.
Obj. 3: Further, Penance comes after Baptism, since it is a second plank, as stated above (A. 6). Now it seems that our Lord instituted Penance before Baptism, because we read that at the beginning of His preaching He said (Matt. 4:17): "Do penance, for the kingdom of heaven is at hand." Therefore this sacrament was not suitably instituted in the New Law.
Obj. 4: Further, the sacraments of the New Law were instituted by Christ, by Whose power they work, as stated above (Q. 62, A. 5; Q. 64, A. 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 (A. 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.