The chapter begins: See how great a love the Father has given to us, that we should be called children of God! The world does not recognize believers, because it didn’t know him. Now they are children of God. It is not yet revealed what they will be, but they know that when he is revealed, they will be like him, for they will see him just as he is. Everyone who has this hope set on him purifies himself, even as he is pure.
The writer contrasts sin and righteousness. Sin is lawlessness. Jesus was revealed to take away sins, and no sin is in him. To this end the Son of God was revealed, that he might destroy the works of the devil. Those born of God do not keep living in sin. The children of God are shown by righteousness and love, while Cain, who killed his brother, shows the opposite. Love is the old message: that we should love one another. We know that we have passed out of death into life, because we love the brothers.
Love has a pattern. By this we know love, because he laid down his life for us. And we ought to lay down our lives for the brothers. Whoever has goods and closes his heart to a brother in need does not have God’s love remaining in him. My little children, let’s not love in word only, or with the tongue only, but in deed and truth. Even when our heart condemns us, God is greater than our heart and knows all things. His commandment is that we believe in the name of his Son Jesus Christ and love one another.