If you are a child of God two things will be evident. 1) You begin believing differently, and 2) You begin behaving differently. If you continue to make a practice of sin in your life you do not belong to God. This litmus test of love is as true for our human relationships, as it is true for our heavenly relationship. If you say you love me, but continue sinning against me, you're lying. In a very real sense, John is echoing the words of James "show me your faith by what you do".