This legalistic belief is based on 1 John 1:9:
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
The key word is IF. If we confess, we're forgiven. Thus, the idea is, if we don't confess, we are doomed. The minute we sin, we lose fellowship (are separated) from God until restored by confession and repentance of the wrong we did. A person who theoretically dies between those two points will be eternally lost. If we sin again five minutes later, the whole process repeats itself. The problem is it allows no room for God’s mercy. When you are saved, all of your sins—past and future—are forgiven:
He, having offered one sacrifice for sins for all time… (Heb 10:12)
When you were dead in your sins and in the uncircumcision of your sinful nature, God made you alive with Christ. He forgave us all our sins. (Col 2:13)
The person who argued with me on this issue pointed out, “It still says if don't confess, we're not forgiven.” But actually, the word confess literally means to agree with. In other words, if we don't agree with God that we have sin, He's not going to forgive us.
Let’s not forget about sins of ignorance. Have you offended someone and didn’t know it? What makes these unknown sins less severe than conscience ones? The former sin goes un-confessed while the latter is confessed. Note Paul’s words:
I am conscious of nothing against myself, yet I am not by this acquitted; but the one who examines me is the Lord. Therefore do not go on passing judgment before the time, but wait until the Lord comes who will both bring to light the things hidden in the darkness and disclose the motives of men’s hearts; and then each man’s praise will come to him from God. (1 Cor 4:4-5)
God is greater than our heart. (1 John 3:19) There are things within us that are ugly, which we often try and cover up or rationalize. They are just as wrong as the most heinous sins.
God’s mercy and forgiveness isn’t based on a scripted legalistic formula. We live by the Spirit, not by rules and regulations—the whole theme of Galatians. Thank God for His great mercy and lovingkindness, so that we can rely on Him. We certainly cannot trust ourselves.