Objection 1. It would seem that the sacrament of Penance should not be repeated. For the Apostle says (Hebrews 6:4, seqq.): "It is impossible for those, who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost... and are fallen away, to be renewed again to penance." Now whosoever have done penance, have been illuminated, and have received the gift of the Holy Ghost. Therefore whosoever sin after doing penance, cannot do penance again.
Objection 2. Further, Ambrose says (De Poenit. ii): "Some are to be found who think they ought often to do penance, who take liberties with Christ: for if they were truly penitent, they would not think of doing penance over again, since there is but one Penance even as there is but one Baptism." Now Baptism is not repeated. Neither, therefore, is Penance to be repeated.
Objection 3. Further, the miracles whereby our Lord healed bodily diseases, signify the healing of spiritual diseases, whereby men are delivered from sins. Now we do not read that our Lord restored the sight to any blind man twice, or that He cleansed any leper twice, or twice raised any dead man to life. Therefore it seems that He does not twice grant pardon to any sinner.
Objection 4. Further, Gregory says (Hom. xxxiv in Evang.): "Penance consists in deploring past sins, and in not committing again those we have deplored": and Isidore says (De Summo Bono ii): "He is a mocker and no penitent who still does what he has repented of." If, therefore, a man is truly penitent, he will not sin again. Therefore Penance cannot be repeated.
Objection 5. Further, just as Baptism derives its efficacy from the Passion of Christ, so does Penance. Now Baptism is not repeated, on account of the unity of Christ's Passion and death. Therefore in like manner Penance is not repeated.
Objection 6. Further, Ambrose says on Psalm 118:58, "I entreated Thy face," etc., that "facility of obtaining pardon is an incentive to sin." If, therefore, God frequently grants pardon through Penance, it seems that He affords man an incentive to sin, and thus He seems to take pleasure in sin, which is contrary to His goodness. Therefore Penance cannot be repeated.
On the contrary, Man is induced to be merciful by the example of Divine mercy, according to Luke 6:36: "Be ye... merciful, as your Father also is merciful." Now our Lord commanded His disciples to be merciful by frequently pardoning their brethren who had sinned against them; wherefore, as related in Matthew 18:21, when Peter asked: "How often shall my brother off end against me, and I forgive him? till seven times?" Jesus answered: "I say not to thee, till seven times, but till seventy times seven times." Therefore also God over and over again, through Penance, grants pardon to sinners, especially as He teaches us to pray (Matthew 6:12): "Forgive us our trespasses, as we forgive them that trespass against us."
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 II-II, 24, 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 Psalm 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 (Genesis 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 Chronicles 33 [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.
Reply to Objection 1. Some of the Jews thought that a man could be washed several times in the laver of Baptism, because among them the Law prescribed certain washing-places where they were wont to cleanse themselves repeatedly from their uncleannesses. In order to disprove this the Apostle wrote to the Hebrews that "it is impossible for those who were once illuminated," viz. through Baptism, "to be renewed again to penance," viz. through Baptism, which is "the laver of regeneration, and renovation of the Holy Ghost," as stated in Titus 3:5: and he declares the reason to be that by Baptism man dies with Christ, wherefore he adds (Hebrews 6:6): "Crucifying again to themselves the Son of God."
Reply to Objection 2. Ambrose is speaking of solemn Penance, which is not repeated in the Church, as we shall state further on (XP, 28, 2).
Reply to Objection 3. As Augustine says [De vera et falsa Poenitentia the authorship of which is unknown, "Our Lord gave sight to many blind men at various times, and strength to many infirm, thereby showing, in these different men, that the same sins are repeatedly forgiven, at one time healing a man from leprosy and afterwards from blindness. For this reason He healed so many stricken with fever, so many feeble in body, so many lame, blind, and withered, that the sinner might not despair; for this reason He is not described as healing anyone but once, that every one might fear to link himself with sin; for this reason He declares Himself to be the physician welcomed not of the hale, but of the unhealthy. What sort of a physician is he who knows not how to heal a recurring disease? For if a man ail a hundred times it is for the physician to heal him a hundred times: and if he failed where others succeed, he would be a poor physician in comparison with them."
Reply to Objection 4. Penance is to deplore past sins, and, "while deploring them," not to commit again, either by act or by intention, those which we have to deplore. Because a man is a mocker and not a penitent, who, "while doing penance," does what he repents having done, or intends to do again what he did before, or even commits actually the same or another kind of sin. But if a man sin afterwards either by act or intention, this does not destroy the fact that his former penance was real, because the reality of a former act is never destroyed by a subsequent contrary act: for even as he truly ran who afterwards sits, so he truly repented who subsequently sins.
Reply to Objection 5. Baptism derives its power from Christ's Passion, as a spiritual regeneration, with a spiritual death, of a previous life. Now "it is appointed unto man once to die" (Hebrews 9:27), and to be born once, wherefore man should be baptized but once. On the other hand, Penance derives its power from Christ's Passion, as a spiritual medicine, which can be repeated frequently.
Reply to Objection 6. According to Augustine (De vera et falsa Poenitentia, the authorship of which is unknown), "it is evident that sins displease God exceedingly, for He is always ready to destroy them, lest what He created should perish, and what He loved be lost," viz. by despair.
Source: Summa Theologica (New Advent)