I answer that, As regards Penance, some have erred, saying that a man cannot obtain pardon of his sins through Penance a second time. Some of these, viz. the Novatians, went so far as to say that he who sins after the first Penance which is done in Baptism, cannot be restored again through Penance. There were also other heretics who, as Augustine relates in De Poenitentia [*De vera et falsa Poenitentia, the authorship of which is unknown], said that, after Baptism, Penance is useful, not many times, but only once.
These errors seem to have arisen from a twofold source: first from not knowing the nature of true Penance. For since true Penance requires charity, without which sins are not taken away, they thought that charity once possessed could not be lost, and that, consequently, Penance, if true, could never be removed by sin, so that it should be necessary to repeat it. But this was refuted in the SS, Question [24], Article [11], where it was shown that on account of free-will charity, once possessed, can be lost, and that, consequently, after true Penance, a man can sin mortally. Secondly, they erred in their estimation of the gravity of sin. For they deemed a sin committed by a man after he had received pardon, to be so grave that it could not be forgiven. In this they erred not only with regard to sin which, even after a sin has been forgiven, can be either more or less grievous than the first, which was forgiven, but much more did they err against the infinity of Divine mercy, which surpasses any number and magnitude of sins, according to Ps. 50:1,2: "Have mercy on me, O God, according to Thy great mercy: and according to the multitude of Thy tender mercies, blot out my iniquity." Wherefore the words of Cain were reprehensible, when he said (Gn. 4:13): "My iniquity is greater than that I may deserve pardon." And so God's mercy, through Penance, grants pardon to sinners without any end, wherefore it is written (2 Paral 37 [*Prayer of Manasses, among the Apocrypha. St. Thomas is evidently quoting from memory, and omits the words in brackets.]): "Thy merciful promise is unmeasurable and unsearchable . . . (and Thou repentest) for the evil brought upon man." It is therefore evident that Penance can be repeated many times.