Again, I write a new commandment to you, which is true in him and in you; because the darkness is passing away, and the true light already shines.
This light was dispelling the darkness of sin and would continue to do so until the final increase of that light will result in the complete annihilation of darkness. When Jesus Christ issued the great commandment anew He called it a new commandment even though God had given it previously (Leviticus 19:18). Now it was important in a new sense due to His coming as the Light of the World (John 13:34-35).