Objection 1. It would seem that by God's mercy all punishment of the damned, both men and demons, comes to an end. For it is written (Wisdom 11:24): "Thou hast mercy upon all, O Lord, because Thou canst do all things." But among all things the demons also are included, since they are God's creatures. Therefore also their punishment will come to an end.
Objection 2. Further, "God hath concluded all in sin [Vulgate: 'unbelief'], that He may have mercy on all" (Romans 11:32). Now God has concluded the demons under sin, that is to say, He permitted them to be concluded. Therefore it would seem that in time He has mercy even on the demons.
Objection 3. Further, as Anselm says (Cur Deus Homo ii), "it is not just that God should permit the utter loss of a creature which He made for happiness." Therefore, since every rational creature was created for happiness, it would seem unjust for it to be allowed to perish altogether.
On the contrary, It is written (Matthew 25:41): "Depart from Me, you cursed, into everlasting fire, which is prepared for the devil and his angels." Therefore they will be punished eternally.
Further, just as the good angels were made happy through turning to God, so the bad angels were made unhappy through turning away from God. Therefore if the unhappiness of the wicked angels comes at length to an end, the happiness of the good will also come to an end, which is inadmissible.
I answer that, As Augustine says (De Civ. Dei xxi) Origen [Cf. I, 64, 2] "erred in maintaining that the demons will at length, through God's mercy, be delivered from their punishment." But this error has been condemned by the Church for two reasons. First because it is clearly contrary to the authority of Holy Writ (Apocalypse 20:9-10): "The devil who seduced them was cast into the pool of fire and brimstone, where both the beasts and the false prophets [Vulgate: 'the beast and false prophet,' etc.] shall be tormented day and night for ever and ever," which is the Scriptural expression for eternity. Secondly, because this opinion exaggerated God's mercy in one direction and depreciated it in another. For it would seem equally reasonable for the good angels to remain in eternal happiness, and for the wicked angels to be eternally punished. Wherefore just as he maintained that the demons and the souls of the damned are to be delivered at length from their sufferings, so he maintained that the angels and the souls of the blessed will at length pass from their happy state to the unhappiness of this life.
Reply to Objection 1. God, for His own part, has mercy on all. Since, however, His mercy is ruled by the order of His wisdom, the result is that it does not reach to certain people who render themselves unworthy of that mercy, as do the demons and the damned who are obstinate in wickedness. And yet we may say that even in them His mercy finds a place, in so far as they are punished less than they deserve condignly, but not that they are entirely delivered from punishment.
Reply to Objection 2. In the words quoted the distribution (of the predicate) regards the genera and not the individuals: so that the statement applies to men in the state of wayfarer, inasmuch as He had mercy both on Jews and on Gentiles, but not on every Gentile or every Jew.
Reply to Objection 3. Anselm means that it is not just in the sense of becoming God's goodness, and is speaking of the creature generically. For it becomes not the Divine goodness that a whole genus of creature fail of the end for which it was made: wherefore it is unbecoming for all men or all angels to be damned. But there is no reason why some men or some angels should perish for ever, because the intention of the Divine will is fulfilled in the others who are saved.
Source: Summa Theologica (New Advent)