I admit that I struggled wording the previous fact into a question.  Everyone survived, so it still works.  When John says you know a person is a Christian based on whether or not they sin, you know it’s not talking about sinless perfection, because of 1 John 1:9.  This is the idea of a continual sinning.  It has the idea that is mentioned in places like Hebrews 12, teaching that a believer will either demonstrate his faith as it naturally results in the good fruit talked about in James, or that Christian will feel the rebuke of the Heaven Father.  However, a person that claims to be a follower of Christ while still living in sin without conviction is someone that is probably not saved.

I hope I explained that well for you.  How about some Old Testament trivia?  What murderer does John mention?