If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
Confession is the acknowledgment of our own short comings.
The message of repentance and confession is the thread of Scripture, especially in the New Testament. If my people, who belong to me, humble themselves, pray, seek to please me, and repudiate their sinful practices, then I will respond from heaven, forgive their sin, and heal their land. The point to make here is that the same message is present in the New Testament. Down the dusty road comes John the Baptist preaching this same message of repentance. But after me will come one who is more powerful than I, whose sandals I am not fit to carry.