In 1 Corinthians 4, St. Paul writes:
But with me it is a very small thing that I should be judged by you or by any human court. I do not even judge myself. I am not aware of anything against myself, but I am not thereby acquitted. It is the Lord who judges me. Therefore do not pronounce judgment before the time, before the Lord comes, who will bring to light the things now hidden in darkness and will disclose the purposes of the heart. Then every man will receive his commendation from God.
Haydock’s Catholic Bible Commentary explains Paul’s meaning:
For I am not conscious. This great apostle of the Gentiles, though conscious to himself of no breach of duty, still does not dare to call himself just. How different is the conduct of this apostle, from those wicked impostors, who teach, that a man is justified by believing himself so. (Estius) — If this privileged apostle was afraid to form any judgment of his own heart and thoughts, whether they were pure or not, but left the trial thereof to the day of judgment, the day of his death, how presumptuous are they, who dare to pronounce on their election and predestination!
Rather than declare himself justified before the Lord and therefore certain of his eternal salvation, St. Paul is content to persevere in his mission and leave to God the judgment concerning his fate.
This doesn’t mean Catholics are not confident that God keeps his promises. It means simply that we must exercise caution. As our tract on salvation states, “Assurance we may have; infallible certitude we may not.”
Scripture teaches that one’s final salvation depends on the state of the soul at death. As Jesus himself tells us, "He who endures to the end will be saved" (Matt. 24:13; cf. 25:31–46). One who dies in the state of friendship with God (the state of grace) will go to heaven. The one who dies in a state of enmity and rebellion against God (the state of mortal sin) will go to hell.
It is usually argued that:
No wrong act or sinful deed can ever affect the believer’s salvation. The sinner did nothing to merit God’s grace and likewise he can do nothing to demerit grace. True, sinful conduct always lessens one’s fellowship with Christ, limits his contribution to God’s work and can result in serious disciplinary action by the Holy Spirit."
However this is not even how things work in everyday life. If another person gives us something as a grace—as a gift—and even if we did nothing to deserve it, it in no way follows that our actions are irrelevant to whether or not we keep the gift. We can lose it in all kinds of ways. We can misplace it, destroy it, give it to someone else, take it back to the store. We may even forfeit something we were given by later displeasing the one who gave it—as when a person has been appointed to a special position but is later stripped of that position on account of mismanagement.
The argument fares no better when one turns to Scripture, for one finds that Adam and Eve, who received God’s grace in a manner just as unmerited as anyone today, most definitely did demerit it—and lost grace not only for themselves but for us as well (cf. also Rom. 11:17-24). While the idea that what is received without merit cannot be lost by demerit may have a kind of poetic charm for some, it does not stand up when compared with the way things really work—either in the everyday world or in the Bible.
Whether Christians have an "absolute" assurance of salvation, regardless of their actions, consider this what Paul said: "See then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness; otherwise you too will be cut off" (Rom. 11:22; see also Heb. 10:26–29, 2 Pet. 2:20–21).