They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 John says two things about sin. First, he has just said that sin is universal; anyone who says that he has not sinned is a liar. Second, there is forgiveness of sins through what Jesus Christ has done, and still does, for men. Now it would be possible to use both these statements as an excuse to think lightly of sin. If all have sinned, why make a fuss about it and what is the use of struggling against something which is in any event an inevitable part of the human situation? Again, if there is forgiveness of sins, why worry about it? 