One of the great purposes of John's letter is that this troubled church obey Christ's commandments (cf. 1 John 1:4; 5:13), a great calling for Christians.  John reassures them that their regard for Christ's law was further proof that they know Him.  They weren't to expect perfect obedience (cf. 1 John 1:6,8,10) but a sincere striving.  After all, Christ is a Savior, an Advocate in heaven who is committed to propitiating sin.