Objection 1. It would seem that at least the punishment of Christians is brought to an end by the mercy of God. "For he that believeth and is baptized shall be saved" (Mark 16:16). Now this applies to every Christian. Therefore all Christians will at length be saved.
Objection 2. Further, it is written (John 6:55): "He that eateth My body and drinketh My blood hath eternal life." Now this is the meat and drink whereof Christians partake in common. Therefore all Christians will be saved at length.
Objection 3. Further, "If any man's work burn, he shall suffer loss: but he himself shall be saved, yet so as by fire" (1 Corinthians 3:15), where it is a question of those who have the foundation of the Christian faith. Therefore all such persons will be saved in the end.
On the contrary, It is written (1 Corinthians 6:9): "The unjust shall not possess the kingdom of God." Now some Christians are unjust. Therefore Christians will not all come to the kingdom of God, and consequently they will be punished for ever.
Further, it is written (2 Peter 2:21): "It had been better for them not to have known the way of justice, than after they have known it, to turn back from that holy commandment which was delivered to them." Now those who know not the way of truth will be punished for ever. Therefore Christians who have turned back after knowing it will also be punished for ever.
I answer that, According to Augustine (De Civ. Dei xxi, 20,21), there have been some who predicted a delivery from eternal punishment not for all men, but only for Christians, although they stated the matter in different ways. For some said that whoever received the sacraments of faith would be immune from eternal punishment. But this is contrary to the truth, since some receive the sacraments of faith, and yet have not faith, without which "it is impossible to please God" (Hebrews 11:6). Wherefore others said that those alone will be exempt from eternal punishment who have received the sacraments of faith, and professed the Catholic faith. But against this it would seem to be that at one time some people profess the Catholic faith, and afterwards abandon it, and these are deserving not of a lesser but of a greater punishment, since according to 2 Peter 2:21, "it had been better for them not to have known the way of justice than, after they have known it, to turn back." Moreover it is clear that heresiarchs who renounce the Catholic faith and invent new heresies sin more grievously than those who have conformed to some heresy from the first. And therefore some have maintained that those alone are exempt from eternal punishment, who persevere to the end in the Catholic faith, however guilty they may have been of other crimes. But this is clearly contrary to Holy Writ, for it is written (James 2:20): "Faith without works is dead," and (Matthew 7:21) "Not every one that saith to Me, Lord, Lord, shall enter into the kingdom of heaven: but he that doth the will of My Father Who is in heaven": and in many other passages Holy Scripture threatens sinners with eternal punishment. Consequently those who persevere in the faith unto the end will not all be exempt from eternal punishment, unless in the end they prove to be free from other crimes.
Reply to Objection 1. Our Lord speaks there of formed faith [Cf. II-II, 4, 3] "that worketh by love [Vulgate: 'charity'; Galatians 5:6]": wherein whosoever dieth shall be saved. But to this faith not only is the error of unbelief opposed, but also any mortal sin whatsoever.
Reply to Objection 2. The saying of our Lord refers not to those who partake only sacramentally, and who sometimes by receiving unworthily "eat and drink judgment" to themselves (1 Corinthians 11:29), but to those who eat spiritually and are incorporated with Him by charity, which incorporation is the effect of the sacramental eating, in those who approach worthily [Cf. III, 80, 1,2,3]. Wherefore, so far as the power of the sacrament is concerned, it brings us to eternal life, although sin may deprive us of that fruit, even after we have received worthily.
Reply to Objection 3. In this passage of the Apostle the foundation denotes formed faith, upon which whosoever shall build venial sins [Cf. I-II, 89, 2] "shall suffer loss," because he will be punished for them by God; yet "he himself shall be saved" in the end "by fire," either of temporal tribulation, or of the punishment of purgatory which will be after death.
Source: Summa Theologica (New Advent)