The best way to answer this question is to quote Scripture.
Let us look to the first epistle of John. Chapter 5, verse 13 reads, "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God. "
Thus, we know that there is some assurance we can have. What is it, though? To answer that, we need to look at John’s statement. he says he has written “these things” that we may know we have salvation. Well, what are the “these things” that he has written?
Looking at the preceding part of the letter, we see John telling us:
- If we love one another, God dwelleth in us, and his love is perfected in us. (4:12)
- If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen? (4:20)
- Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God. (4:15)
- Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him. (5:1)
- By this we know that we love the children of God, when we love God, and keep his commandments.(5:2)
This is just a general sample, there are more verses I could have listed. The point is that the things John wrote to us so that we know if we have eternal life are a series of statements about how to identify when we have life.
If we keep God’s commandments, we have life.
If we believe in Christ, we have life. If that, whosoever that, if the other thing. What John is doing here is not telling us that all who believe in the Son of God should be assured they have eternal life, what he is doing is giving us a list of things to look at to help determine we are actually living in Christ, to see if we do have eternal life. This is all in the context of John beginning the letter by telling us that, “If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.”
Reading the entire letter of John through, one can see that the letter is a letter written to believers to encourage believers, help them to grow, and ensure that they really are in salvation. This is why the beginning of the letter begins with an exhortation to confess if we need to and the end tells us to look out for “sin that is unto death.” The entire letter is meant to help believers to remain in Christ, by exhorting them to confess and remain in Christ, giving them advice on how to determine if they are, and warning them about ‘sin unto death’ at the end.
This letter perfectly describes Catholic moral certitude: if we, by God’s Grace, remain in Him, we will have salvation - He will not let us down. The Protestant idea of assurance sortof skips over the first 4 chapters of the epistle and gets straight to “you may know you have eternal life,” saying that we have eternal life
reagardless of those things John spoke about in chapters 3 and 4.