John began his first epistle by reassuring us of the truth of the incarnation of Jesus, but also told us that because of the truth of the incarnation and resurrection we have fellowship with the Father and the Son and one another. It makes all of the sense in the world then that what John has to address next is that which breaks fellowship and what we do about it. Sin breaks fellowship and our temptations with sin is to downplay them, or to deny them, or to despair over. But thanks be to God is not a liar but deals is honest about sin and what has to be done with it. Jesus is the propitiation, the atonement for our sins. He turns away the wrath of God by the gift of his own life for us who break fellowship with him and others.