It's not uncommon to encounter the random street preacher who asks the question, “Are you saved?” He may follow up by asking, "Are you certain?" In fact, my Evangelical Christian friends have asked me this many times.
These questions find their purpose in the belief of most Evangelicals that once you have accepted Jesus as your personal Savior you can be assured of your own salvation. This is not compatible with Catholic teaching.
The Catholic Answers tract on salvation offers a fine response to the question:
"Are you saved?" asks the Fundamentalist. The Catholic should reply: "As the Bible says, I am already saved (Rom. 8:24, Eph. 2:5–8), but I’m also being saved (1 Cor. 1:18, 2 Cor. 2:15, Phil. 2:12), and I have the hope that I will be saved (Rom. 5:9–10, 1 Cor. 3:12–15). Like the apostle Paul I am working out my salvation in fear and trembling (Phil. 2:12), with hopeful confidence in the promises of Christ (Rom. 5:2, 2 Tim. 2:11–13)."
The above is wholly adequate given the opportunity to elaborate on the answer, but I use a different approach in face-to-face situations where my time is limited.
One example of this would be an interdenominational Bible study. I have attended several of theses (a practice I do not recommend unless you already have a decent grasp of Catholic theology and Sacred Scripture) and found that an answer like the one above requires a fair amount of explanation. To avoid the perception that I am trying to dominate the conversation, I will appeal to only one passage from Scripture rather than many verses.
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. (3-5)
Haydock’s Catholic Bible Commentary explains Paul’s meaning:
Ver. 4. 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 we 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.”
Keep in mind I do not expect the above to end a conversation on salvation with my non-Catholic friends. Obviously, much more can be said on the subject. I use it only when I do not have the time for a prolonged conversation, or to avoid appearing as though I am trying to dominate a group discussion.
As is the case in any conversation, you have to make a judgment call: Does the situation allow for a long explanation, or do you just need to plant a seed? If it is the latter, then use the best and least amount of evidence to make your point.