“If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.”
and to cleanse us from all unrighteousness
The word “cleanse” means to make clean, purify. Not only does God forgive us for the sins we confess, but He cleanses us from the sins we forget to confess. If we confess the sins of which we are aware, God will forgive us for the sins of which we are unaware. God cleanses us from anything that would prevent a believer from being right with God.
Note the word “all.” God will never overlook the slightest bit of unrighteousness in us, but He will cleanse the slightest unrighteousness. God cleanses us all overt and covert sins. He cleanses us from conscious sins and unwitting sins. In His cleansing, He takes care of sins of commission and omission.
If we confess our known sins, God will forgive us the sins we forgot to confess.
You have committed hundreds of sins you forgot to confess. What do you do with them? Are you perpetually out of fellowship with God because you forgot to confess them? The point of cleansing us from all unrighteousness is that if we confess the sins we know, God will forgive us of the sins we forgot to confess. This is grace upon grace.