Objection 1. It would seem that an eternal punishment is not inflicted on sinners by Divine justice. For the punishment should not exceed the fault: "According to the measure of the sin shall the measure also of the stripes be" (Deuteronomy 25:2). Now fault is temporal. Therefore the punishment should not be eternal.
Objection 2. Further, of two mortal sins one is greater than the other. and therefore one should receive a greater punishment than the other. But no punishment is greater than eternal punishment, since it is infinite. Therefore eternal punishment is not due to every sin; and if it is not due to one, it is due to none, since they are not infinitely distant from one another.
Objection 3. Further, a just judge does not punish except in order to correct, wherefore it is stated (Ethic. ii, 3) that "punishments are a kind of medicine." Now, to punish the wicked eternally does not lead to their correction, nor to that of others, since then there will be no one in future who can be corrected thereby. Therefore eternal punishment is not inflicted for sins according to Divine justice.
Objection 4. Further, no one wishes that which is not desirable for its own sake, except on account of some advantage. Now God does not wish punishment for its own sake, for He delights not in punishments [The allusion is to Wisdom 1:13: "Neither hath He pleasure in the destruction of the living," as may be gathered from I-II, 87, 3, Objection 3]. Since then no advantage can result from the perpetuity of punishment, it would seem that He ought not to inflict such a punishment for sin.
Objection 5. Further, "nothing accidental lasts for ever" (De Coelo et Mundo i). But punishment is one of those things that happen accidentally, since it is contrary to nature. Therefore it cannot be everlasting.
Objection 6. Further, the justice of God would seem to require that sinners should be brought to naught: because on account of ingratitude a person deserves to lose all benefits, and among other benefits of God there is "being" itself. Therefore it would seem just that the sinner who has been ungrateful to God should lose his being. But if sinners be brought to naught, their punishment cannot be everlasting. Therefore it would seem out of keeping with Divine justice that sinners should be punished for ever.
On the contrary, It is written (Matthew 25:46): "These," namely the wicked, "shall go into everlasting punishment."
Further, as reward is to merit, so is punishment to guilt. Now, according to Divine justice, an eternal reward is due to temporal merit: "Every one who seeth the Son and believeth in Him hath [Vulgate: 'that everyone... may have'] life everlasting." Therefore according to Divine justice an everlasting punishment is due to temporal guilt.
Further, according to the Philosopher (Ethic. v, 5), punishment is meted according to the dignity of the person sinned against, so that a person who strikes one in authority receives a greater punishment than one who strikes anyone else. Now whoever sins mortally sins against God, Whose commandments he breaks, and Whose honor he gives another, by placing his end in some one other than God. But God's majesty is infinite. Therefore whoever sins mortally deserves infinite punishment; and consequently it seems just that for a mortal sin a man should be punished for ever.
I answer that, Since punishment is measured in two ways, namely according to the degree of its severity, and according to its length of time, the measure of punishment corresponds to the measure of fault, as regards the degree of severity, so that the more grievously a person sins the more grievously is he punished: "As much as she hath glorified herself and lived in delicacies, so much torment and sorrow give ye to her" (Apocalypse 18:7). The duration of the punishment does not, however, correspond with the duration of the fault, as Augustine says (De Civ. Dei xxi, 11), for adultery which is committed in a short space of time is not punished with a momentary penalty even according to human laws [Cf. I-II, 87, 3, ad 1]. But the duration of punishment regards the disposition of the sinner: for sometimes a person who commits an offense in a city is rendered by his very offense worthy of being cut off entirely from the fellowship of the citizens, either by perpetual exile or even by death: whereas sometimes he is not rendered worthy of being cut off entirely from the fellowship of the citizens. wherefore in order that he may become a fitting member of the State, his punishment is prolonged or curtailed, according as is expedient for his amendment, so that he may live in the city in a becoming and peaceful manner. So too, according to Divine justice, sin renders a person worthy to be altogether cut off from the fellowship of God's city, and this is the effect of every sin committed against charity, which is the bond uniting this same city together. Consequently, for mortal sin which is contrary to charity a person is expelled for ever from the fellowship of the saints and condemned to everlasting punishment, because as Augustine says (De Civ. Dei xxi, 11), "as men are cut off from this perishable city by the penalty of the first death, so are they excluded from that imperishable city by the punishment of the second death." That the punishment inflicted by the earthly state is not deemed everlasting is accidental, either because man endures not for ever, or because the state itself comes to an end. Wherefore if man lived for ever, the punishment of exile or slavery, which is pronounced by human law, would remain in him for ever. On the other hand, as regards those who sin in such a way as not to deserve to be entirely cut off from the fellowship of the saints, such as those who sin venially, their punishment will be so much the shorter or longer according as they are more or less fit to be cleansed, through sin clinging to them more or less: this is observed in the punishments of this world and of purgatory according to Divine justice.
We find also other reasons given by the saints why some are justly condemned to everlasting punishment for a temporal sin. One is because they sinned against an eternal good by despising eternal life. This is mentioned by Augustine (De Civ. Dei. xii, 12): "He is become worthy of eternal evil, who destroyed in himself a good which could be eternal." Another reason is because man sinned in his own eternity [Cf. I-II, 87, 3, ad 1]; wherefore Gregory says (Dial. iv), it belongs to the great justice of the judge that those should never cease to be punished, who in this life never ceased to desire sin. And if it be objected that some who sin mortally propose to amend their life at some time, and that these accordingly are seemingly not deserving of eternal punishment, it must be replied according to some that Gregory speaks of the will that is made manifest by the deed. For he who falls into mortal sin of his own will puts himself in a state whence he cannot be rescued, except God help him: wherefore from the very fact that he is willing to sin, he is willing to remain in sin for ever. For man is "a wind that goeth," namely to sin, "and returneth not by his own power" (Psalm 77:39). Thus if a man were to throw himself into a pit whence he could not get out without help, one might say that he wished to remain there for ever, whatever else he may have thought himself. Another and a better answer is that from the very fact that he commits a mortal sin, he places his end in a creature; and since the whole of life is directed to its end, it follows that for this very reason he directs the whole of his life to that sin, and is willing to remain in sin forever, if he could do so with impunity. This is what Gregory says on Job 41:23, "He shall esteem the deep as growing old" (Moral. xxxiv): "The wicked only put an end to sinning because their life came to an end: they would indeed have wished to live for ever, that they might continue in sin for ever for they desire rather to sin than to live." Still another reason may be given why the punishment of mortal sin is eternal: because thereby one offends God Who is infinite. Wherefore since punishment cannot be infinite in intensity, because the creature is incapable of an infinite quality, it must needs be infinite at least in duration. And again there is a fourth reason for the same: because guilt remains for ever, since it cannot be remitted without grace, and men cannot receive grace after death; nor should punishment cease so long as guilt remains.
Reply to Objection 1. Punishment has not to be equal to fault as to the amount of duration as is seen to be the case also with human laws. We may also reply with Gregory (Dial. xliv) that although sin is temporal in act, it is eternal in will.
Reply to Objection 2. The degree of intensity in the punishment corresponds to the degree of gravity in the sin; wherefore mortal sins unequal in gravity will receive a punishment unequal in intensity but equal in duration.
Reply to Objection 3. The punishments inflicted on those who are not altogether expelled from the society of their fellow-citizens are intended for their correction: whereas those punishments, whereby certain persons are wholly banished from the society of their fellow-citizens, are not intended for their correction; although they may be intended for the correction and tranquillity of the others who remain in the state. Accordingly the damnation of the wicked is for the correction of those who are now in the Church; for punishments are intended for correction, not only when they are being inflicted, but also when they are decreed.
Reply to Objection 4. The everlasting punishment of the wicked will not be altogether useless. For they are useful for two purposes. First, because thereby the Divine justice is safeguarded which is acceptable to God for its own sake. Hence Gregory says (Dial. iv): "Almighty God on account of His loving kindness delights not in the torments of the unhappy, but on account of His justice. He is for ever unappeased by the punishment of the wicked." Secondly, they are useful, because the elect rejoice therein, when they see God's justice in them, and realize that they have escaped them. Hence it is written (Psalm 57:12): "The just shall rejoice when he shall see the revenge," etc., and (Isaiah 66:24): "They," namely the wicked, "shall be a loathsome sight* to all flesh," namely to the saints, as a gloss says. ["Ad satietatem visionis," which St. Thomas takes to signify being satiated with joy; Cf. 94, 3]. Gregory expresses himself in the same sense (Dial. iv): "The wicked are all condemned to eternal punishment, and are punished for their own wickedness. Yet they will burn to some purpose, namely that the just may all both see in God the joys they receive, and perceive in them the torments they have escaped: for which reason they will acknowledge themselves for ever the debtors of Divine grace the more that they will see how the evils which they overcame by its assistance are punished eternally."
Reply to Objection 5. Although the punishment relates to the soul accidentally, it relates essentially to the soul infected with guilt. And since guilt will remain in the soul for ever, its punishment also will be everlasting.
Reply to Objection 6. Punishment corresponds to fault, properly speaking, in respect of the inordinateness in the fault, and not of the dignity in the person offended: for if the latter were the case, a punishment of infinite intensity would correspond to every sin. Accordingly, although a man deserves to lose his being from the fact that he has sinned against God the author of his being, yet, in view of the inordinateness of the act itself, loss of being is not due to him, since being is presupposed to merit and demerit, nor is being lost or corrupted by the inordinateness of sin [Cf. I-II, 85, 1]: and consequently privation of being cannot be the punishment due to any sin.
Source: Summa Theologica (New Advent)