1 John was written to believers, to Christians, to help them in their walk of fellowship with the Lord. All of our sins are forgiven when we put our faith in Christ and we are given an eternal relationship with God as His children. However, there is still the need for daily cleansing in the believer’s life in order to have fellowship with the Lord. Confession is for daily fellowship.
Known sin in the life of a believer grieves the Holy Spirit and quenches His power (Eph. 4:30; 1 Thess. 5:19). But honest-to-God, sincere confession brings forgiveness from the standpoint of abiding in the Lord or walking in fellowship with Him. Thus, when we confess our sins, God forgives us from the sins we confess PLUS He cleanses us from all our unknown sins. Our series, ABCs for Christian Growth will explain this in much more detail.