A -Thanks for the question. You are correct in your thinking and some distinctions need to be made. First, not all Protestants believe what your friend believes - "once saved, always saved". This is a belief that grew out of the Reformed tradition of Protestantism, which was begun by John Calvin. He believed that if a Christian accepted Christ's salvation, then there was no sin or work that we could do to resist that grace or change it's effects on our souls. We are destined for heaven regardless of everything else. But, this is simply not a Catholic understanding of salvation or what the Bible says about salvation.
The Catholic belief is that salvation is a process that continues until we die and are judged by God. We will then have the process of salvation completed in us. Where do we get this belief? From the Bible. It talks about three aspects to the journey of salvation - past, present, and future. We were saved by Christ dying for our sins. We are being saved now as we journey with Christ. We will be saved once we die and enter heaven.
Here are some Scriptures to show this journey:
- "it is by grace you have been saved." - Eph 2:5
- "He saved us and called us through grace and not by virtue of our own works outside of His grace." - 2 Tim. 1:9
- "He saved us in virtue of His own mercy, and not by our deeds." - Titus 3:5
- "For we are to God the aroma of Christ among those who are being saved" - 2 Cor 2:15
- "Continue to work out your salvation with fear and trembling" - Phil. 2:12
- "for you are receiving the goal of your faith, the salvation of your souls." - 1 Peter 1:9
- "The hour has come for you to wake up from your slumber, because our salvation is nearer now than when we first believed." - Romans 13:11
- "if we endure, we will also reign with him. If we disown him, he will also disown us;" - 2 tim 2:12
- "he who stands firm to the end will be saved." - Matt 10:22
We can see the progression of salvation in Scripture. With this being said, the Bible also teaches that salvation can be lost once gained.
- "I beat my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize." - 1 Cor 9:27
- "Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain." - 1 Cor 15: 1-2
- "The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons." - 1 Tim 4:1
- "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace." - Heb 6:4-6
Even St. Paul talks about not judging his own soul, but allowing God to do so.
- "My conscience is clear, but that does not make me innocent. It is the Lord who judges me." - 1 Cor 4:4
Now, someone might ask why a Catholic wouldn't fall into despair if we don't have a certain assurance of salvation. But, a Catholic should neither despair that God will abandon us (knowing He would never abandon us) but rather we wouldn't presume that God will save us if we decide we don't want Him to.
Right now, because I know I am in the state of grace, if I were to die, I can have a "moral assurance" I would go to heaven. But, I cannot say the same thing for my future state. This does not lead me to despair - but to rightly hope in God's salvation for me.
If you talk to your friend about this, don't try to prove them wrong. We don't want to destroy their faith, but build upon the truth already there. So, you might dialogue about what Sacred Scripture says about the process of salvation - but don't get into a "verse slinging" contest either.
Here is a similar post I made about this topic previously.