Objection 1. It would seem that all who perform works of mercy will not be punished eternally, but only those who neglect those works. For it is written (James 2:13): "Judgment without mercy to him that hath not done mercy"; and (Matthew 5:7): "Blessed are the merciful for they shall obtain mercy."
Objection 2. Further, (Matthew 25:35-46) we find a description of our Lord's discussion with the damned and the elect. But this discussion is only about works of mercy. Therefore eternal punishment will be awarded only to such as have omitted to practice works of mercy: and consequently the same conclusion follows as before.
Objection 3. Further, it is written (Matthew 6:12): "Forgive us our debts, as we also forgive our debtors," and further on (Matthew 6:14): "For if you will forgive men their offenses, your heavenly Father will forgive you also your offenses." Therefore it would seem that the merciful, who forgive others their offenses, will themselves obtain the forgiveness of their sins, and consequently will not be punished eternally.
Objection 4. Further, a gloss of Ambrose on 1 Timothy 4:8, "Godliness is profitable to all things," says: "The sum total of a Christian's rule of life consists in mercy and godliness. Let a man follow this, and though he should suffer from the inconstancy of the flesh, without doubt he will be scourged, but he will not perish: whereas he who can boast of no other exercise but that of the body will suffer everlasting punishment." Therefore those who persevere in works of mercy, though they be shackled with fleshly sins, will not be punished eternally: and thus the same conclusion follows as before.
On the contrary, It is written (1 Corinthians 6:9-10): "Neither fornicators... nor adulterers," etc. "shall possess the kingdom of God." Yet many are such who practice works of mercy. Therefore the merciful will not all come to the eternal kingdom: and consequently some of them will be punished eternally.
Further, it is written (James 2:10): "Whosoever shall keep the whole law, but offend in one point, is become guilty of all." Therefore whoever keeps the law as regards the works of mercy and omits other works, is guilty of transgressing the law, and consequently will be punished eternally.
I answer that, As Augustine says in the book quoted above (De Civ. Dei xxi, 22), some have maintained that not all who have professed the Catholic faith will be freed from eternal punishment, but only those who persevere in works of mercy, although they be guilty of other crimes. But this cannot stand, because without charity nothing can be acceptable to God, nor does anything profit unto eternal life in the absence of charity. Now it happens that certain persons persevere in works of mercy without having charity. Wherefore nothing profits them to the meriting of eternal life, or to exemption from eternal punishment, as may be gathered from 1 Corinthians 13:3. Most evident is this in the case of those who lay hands on other people's property, for after seizing on many things, they nevertheless spend something in works of mercy. We must therefore conclude that all whosoever die in mortal sin, neither faith nor works of mercy will free them from eternal punishment, not even after any length of time whatever.
Reply to Objection 1. Those will obtain mercy who show mercy in an ordinate manner. But those who while merciful to others are neglectful of themselves do not show mercy ordinately, rather do they strike at themselves by their evil actions. Wherefore such persons will not obtain the mercy that sets free altogether, even if they obtain that mercy which rebates somewhat their due punishment.
Reply to Objection 2. The reason why the discussion refers only to the works of mercy is not because eternal punishment will be inflicted on none but those who omit those works, but because eternal punishment will be remitted to those who after sinning have obtained forgiveness by their works of mercy, making unto themselves "friends of the mammon of iniquity" (Luke 16:9).
Reply to Objection 3. Our Lord said this to those who ask that their debt be forgiven, but not to those who persist in sin. Wherefore the repentant alone will obtain by their works of mercy the forgiveness that sets them free altogether.
Reply to Objection 4. The gloss of Ambrose speaks of the inconstancy that consists in venial sin, from which a man will be freed through the works of mercy after the punishment of purgatory, which he calls a scourging. Or, if he speaks of the inconstancy of mortal sin, the sense is that those who while yet in this life fall into sins of the flesh through frailty are disposed to repentance by works of mercy. Wherefore such a one will not perish, that is to say, he will be disposed by those works not to perish, through grace bestowed on him by our Lord, Who is blessed for evermore. Amen.
Source: Summa Theologica (New Advent)