This is how we know that we know him: if we keep his commandments.

 To this day the Christian cannot escape the obligation to be different from the world. In this passage John sees things as he always sees them--in terms of black and white. As Westcott has it: "There cannot be a vacuum in the soul." This is a matter in which there is no neutrality; a man either loves the world or he loves God, Jesus himself said, "No one can serve two masters" ( Matthew 6:24). The ultimate choice remains the same. Are we to accept the world's standards or the standards of God? 