Objection 1. It would seem that God's mercy does not suffer at least men to be punished eternally. For it is written (Genesis 6:3): "My spirit shall not remain in man for ever because he is flesh"; where "spirit" denotes indignation, as a gloss observes. Therefore, since God's indignation is not distinct from His punishment, man will not be punished eternally.
Objection 2. Further, the charity of the saints in this life makes them pray for their enemies. Now they will have more perfect charity in that life. Therefore they will pray then for their enemies who are damned. But the prayers of the saints cannot be in vain, since they are most acceptable to God. Therefore at the saints' prayers the Divine mercy will in time deliver the damned from their punishment.
Objection 3. Further, God's foretelling of the punishment of the damned belongs to the prophecy of commination. Now the prophecy of commination is not always fulfilled: as appears from what was said of the destruction of Nineve (Jonah 3); and yet it was not destroyed as foretold by the prophet, who also was troubled for that very reason (Jonah 4:1). Therefore it would seem that much more will the threat of eternal punishment be commuted by God's mercy for a more lenient punishment, when this will be able to give sorrow to none but joy to all.
Objection 4. Further, the words of Psalm 76:8 are to the point, where it is said: "Will God then be angry for ever? [Vulgate: 'Will God then cast off for ever?']" But God's anger is His punishment. Therefore, etc.
Objection 5. Further, a gloss on Isaiah 14:19, "But thou art cast out," etc. says: "Even though all souls shall have rest at last, thou never shalt": and it refers to the devil. Therefore it would seem that all human souls shall at length have rest from their pains.
On the contrary, It is written (Matthew 25:46) of the elect conjointly with the damned: "These shall go into everlasting punishment: but the just, into life everlasting." But it is inadmissible that the life of the just will ever have an end. Therefore it is inadmissible that the punishment of the damned will ever come to an end.
Further, as Damascene says (De Fide Orth. ii) "death is to men what their fall was to the angels." Now after their fall the angels could not be restored [Cf. I, 64, 2]. Therefore neither can man after death: and thus the punishment of the damned will have no end.
I answer that, As Augustine says (De Civ. Dei xxi, 17,18), some evaded the error of Origen by asserting that the demons are punished everlastingly, while holding that all men, even unbelievers, are at length set free from punishment. But this statement is altogether unreasonable. For just as the demons are obstinate in wickedness and therefore have to be punished for ever, so too are the souls of men who die without charity, since "death is to men what their fall was to the angels," as Damascene says.
Reply to Objection 1. This saying refers to man generically, because God's indignation was at length removed from the human race by the coming of Christ. But those who were unwilling to be included or to remain in this reconciliation effected by Christ, perpetuated the Divine anger in themselves, since no other way of reconciliation is given to us save that which is through Christ.
Reply to Objection 2. As Augustine (De Civ. Dei xxi, 24) and Gregory (Moral. xxxiv) say, the saints in this life pray for their enemies, that they may be converted to God, while it is yet possible for them to be converted. For if we knew that they were foreknown to death, we should no more pray for them than for the demons. And since for those who depart this life without grace there will be no further time for conversion, no prayer will be offered for them, neither by the Church militant, nor by the Church triumphant. For that which we have to pray for them is, as the Apostle says (2 Timothy 2:25-26), that "God may give them repentance to know the truth, and they may recover themselves from the snares of the devil."
Reply to Objection 3. A punishment threatened prophetically is only then commuted when there is a change in the merits of the person threatened. Hence: "I will suddenly speak against a nation and against a kingdom, to root out and to pull down and to destroy it. If that nation... shall repent of their evil, I also will repent of the evil that I have thought to do to them" (Jeremiah 18:7). Therefore, since the merits of the damned cannot be changed, the threatened punishment will ever be fulfilled in them. Nevertheless the prophecy of commination is always fulfilled in a certain sense, because as Augustine says (De Civ. Dei. xxi, 24): "Nineve has been overthrown, that was evil, and a good Nineve is built up, that was not: for while the walls and the houses remained standing, the city was overthrown in its wicked ways."
Reply to Objection 4. These words of the Psalm refer to the vessels of mercy, which have not made themselves unworthy of mercy, because in this life (which may be called God's anger on account of its unhappiness) He changes vessels of mercy into something better. Hence the Psalm continues (Psalm 76:11): "This is the change of the right hand of the most High." We may also reply that they refer to mercy as granting a relaxation but not setting free altogether if it be referred also to the damned. Hence the Psalm does not say: "Will He from His anger shut up His mercies?" but "in His anger," because the punishment will not be done away entirely; but His mercy will have effect by diminishing the punishment while it continues.
Reply to Objection 5. This gloss is speaking not absolutely but on an impossible supposition in order to throw into relief the greatness of the devil's sin, or of Nabuchodonosor's.
Source: Summa Theologica (New Advent)