I imagine you’re going to get a pretty big response to this as it is one of the great, historical sticking points between Catholics and most Protestants. I beleive it is safe to say that salvation is not guaranteed for anyone. We can never merit salvation of our own works, but we must *cooperate *with the Grace we are given. Only the Lord knows if we have cooperated to the level He would expect.
Keep in mind St. Paul said, “we must work out our salvation with fear and trembling” (Phillipians 2: 12) If salvation we’re guaranteed, why the fear? Why the trembling? My NIV Bible (the favorite Bible of many Evangelicals including myself when I was one) notes that this fear and trembling has to do with “an active reverence and a singleness of purpose in response to God’s grace.” Quite a stretch if you ask me.
In 1 Corinthians 9: 24-27, we find another reference often used to show that the Catholic understanding of cooperating with Grace is the proper one. St. Paul likens the situation to a race. One in which he fears that he himself may be disqualified. Here the question is simply put, how is it that so many of our Protestant brothers and sisters can know that they are saved, when St. Paul himself, yes the very same one who once persecuted the Christians until hearing the voice of the Lord, seems uncertain in his own regard?
Other passages you might want to look at are:
John 14: 21
Romans 2: 2-8; 11: 22
Matthew 7:21; 19: 16-17; 24: 13
James 2: 14-24
1 Corinthians 10: 11-12
Galatians 5: 4
2 Timothy 2: 11-13
Hebrews 6: 4-6; 10: 26-27
There are certainly others, but these passages might give those outside (and inside) the Church, something to think about.
Always remember, too, that we can throw Bible passages at each other all day. This is a good argument for why we need a Papacy and Magisterium, i.e. a group of ordained teachers who are guided by the Holy Spirit from error. If Christ had not so intended it, we would have no way of knowing who’s interpretations are correct.