Behold, how great a love the Father has bestowed on us, that we should be called children of God! For this cause the world doesn't know us, because it didn't know him.

 (ii) He tells us what sin does. It undoes the work of Christ. Christ is the Lamb of God who takes away the sins of the world ( John 1:29). To sin is to bring back what he came into the world to abolish. 