Objection 1. It would seem that satisfaction need not be made by means of penal works. For satisfaction should make compensation for the offense committed against God. Now, seemingly, no compensation is given to God by penal works, for God does not delight in our sufferings, as appears from Tobit 3:22. Therefore satisfaction need not be made by means of penal works.
Objection 2. Further, the greater the charity from which a work proceeds, the less penal is that work, for "charity hath no pain [Vulgate: 'Perfect charity casteth out fear, because fear hath pain']" according to 1 John 4:18. If therefore works of satisfaction need to be penal, the more they proceed from charity, the less satisfactory will they be: which is false.
Objection 3. Further, "Satisfaction," as Anselm states (Cur Deus homo i) "consists in giving due honor to God." But this can be done by other means than penal works. Therefore satisfaction needs not to be made by means of penal works.
On the contrary, Gregory says (Hom. in Evang. xx): "It is just that the sinner, by his repentance, should inflict on himself so much the greater suffering, as he has brought greater harm on himself by his sin."
Further, the wound caused by sin should be perfectly healed by satisfaction. Now punishment is the remedy for sins, as the Philosopher says (Ethic. ii, 3). Therefore satisfaction should be made by means of penal works.
I answer that, As stated above (Question 12, Article 3), satisfaction regards both the past offense, for which compensation is made by its means, and also future sin wherefrom we are preserved thereby: and in both respects satisfaction needs to be made by means of penal works. For compensation for an offense implies equality, which must needs be between the offender and the person whom he offends. Now equalization in human justice consists in taking away from one that which he has too much of, and giving it to the person from whom something has been taken. And, although nothing can be taken away from God, so far as He is concerned, yet the sinner, for his part, deprives Him of something by sinning as stated above (12, 3,4). Consequently, in order that compensation be made, something by way of satisfaction that may conduce to the glory of God must be taken away from the sinner. Now a good work, as such, does not deprive the agent of anything, but perfects him: so that the deprivation cannot be effected by a good work unless it be penal. Therefore, in order that a work be satisfactory it needs to be good that it may conduce to God's honor, and it must be penal, so that something may be taken away from the sinner thereby.
Again punishment preserves from future sin, because a man does not easily fall back into sin when he has had experience of the punishment. Wherefore, according to the Philosopher (Ethic. ii, 3) punishments are medicinal.
Reply to Objection 1. Though God does not delight in our punishments as such, yet He does, in so far as they are just, and thus they can be satisfactory.
Reply to Objection 2. Just as, in satisfaction, we have to note the penality of the work, so, in merit, we must observe its difficulty. Now if the difficulty of the work itself be diminished, other things being equal, the merit is also diminished; but if the difficulty be diminished on the part of the promptitude of the will, this does not diminish the merit, but increases it; and, in like manner, diminution of the penality of a work, on account of the will being made more prompt by charity, does not lessen the efficacy of satisfaction, but increases it.
Reply to Objection 3. That which is due for sin is compensation for the offense, and this cannot be done without punishment of the sinner. It is of this debt that Anselm speaks.
Source: Summa Theologica (New Advent)