Hebrews 6:11 says: “And we desire that each one of you show the same diligence so as to realize the full assurance of hope until the end.” Though scripture is very clear that salvation is entirely of grace, it also teaches that assurance of that salvation is based, at least to some extent, upon one’s fruit. We see that here in Hebrews 6:
— He says in :9 that he is convinced of better things of them, things that accompany salvation.
— :10 For God is not so unjust as to forget your work
— :10 and the love you have shown towards His name in ministering … to the saints
— :11we desire that each one of you show the same diligence
He then says that if they are doing these things, THEN they will have “full assurance of hope until the end.” Thus their confidence, or assurance, is based upon seeing that God really is at work in them, and they and others can see their fruit.
This is not without reinforcement in other places in scripture: I John 3:19, “We will know by this that we are of the truth, and shall assure our heart before Him …”. Speaking in context of loving others in deed and truth, not just with words, John says — like the author of Hebrews does — that if they are doing these things, they will have assurance of salvation in their hearts.
The opposite is also true. There is no biblical confidence of salvation for a person who can point to a date in the past when they “accepted Christ” but who has no present works which confirm his salvation. Perhaps that person is saved — scripture does speak of “carnal Christians” and backsliders — but there is no ASSURANCE for them. Assurance, in the New Testament, is for those who demonstrate that they are bearing fruit of their profession of faith in Christ. Without that fruit, there can be no real Biblical confidence of salvation.