But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus Christ, his Son, cleanses us from all sin.
Fellowship one with another ... Although not stated in this sentence, this fellowship is also with God the Father and his Son Jesus Christ (1 John 3).  This fellowship stands for membership in the corporate spiritual body of Jesus Christ.  Thus the cleansing here mentioned is not that from "old sins" prior to conversion, but from the accrual of sins daily by the Christian.