Being forgiven by God is part and parcel of what it means to be justified (Ac 13:38-39) or saved (Ro 5:9), to be His child. If we die with any sin which isn’t forgiven by God, which is not washed away by the blood of Christ (Re 1:5), the consequence is spiritual death (Ez 18:4), separation from God forever in Hell. (Ro 6:23) So, it’s vital to ensure that all our sins are forgiven, every last one of them. (Co 2:13)
If we make forgiveness of our sin conditional upon our confession, then by definition this behavior becomes a means of earning or maintaining salvation, since it implies we’re lost forever if we don’t faithfully confess all our sins.
This is effectively salvation by works, or legalism, which the Scripture rejects as inconsistent with the Gospel: we’re justified by faith (Ro 3:28), not by works. (Ga 5:4)
God’s salvation doesn’t require that we act a certain way to keep it; rather, it produces behavior within us reflecting the divine nature; we become new creatures (2Co 5:17), moving away from sin to walk with God.
Striving against sin is an ongoing process where we’re growing over time; no believer is perfectly sinless this side of Heaven (Ro 7:21); we continue to sin in ways we hardly even comprehend (1Jn 1:8), but we don’t commit ourselves to a life of willful, presumptuous sin. (1Jn 3:8-10)
As believers, once we realize we’ve sinned we acknowledge this before God; we confess our sin, agreeing with God about it, and look to Him to help us overcome sin, knowing He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. (1Jn 1:9)
He doesn’t forgive us because we confess our sin, Father God forgives us because Jesus died for us and paid our sin debt once for all. (He 10:14) Confession is how we restore our fellowship with God, how we walk in the light (1Jn 1:7), not how we get Him to forgive us.
Confession does not cause forgiveness; we believers are forgiven of all our sins — past, present and future sins. Confession of sin is simply the natural response for us as we become aware of our sins, and when we do confess we can rest assured we’re forgiven: He never even charges us with sin. (Ro 4:7-8) This is the only way we can be sure of our salvation: God has already accomplished it for us. (Ro 7:16)
We may confirm 1st John 1:9 does not teach this cause-and-effect relationship between confession of sin and forgiveness by carefully noting what it says, and what the text doesn’t say. It says: “If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.” The text does not say, “If we don’t confess our sins, God will not forgive us.”
This latter statement, which many infer from this verse, is called the Inverse and it’s not logically equivalent to the original (it means something different). To illustrate, I might say, “If it rains I will go to the store.” If it doesn’t rain and I do go to the store, I’m not being inconsistent; I didn’t say what I would do if it didn’t rain, only what I would do if it did rain. So, 1st John 1:9 says nothing about what God will do if we don’t confess a particular sin; it simply affirms that as we confess our sins we may be confident we’re forgiven.