John will confirm his doctrine by the principle of love, which was the driving passion which led Jesus ultimately to the cross. He adopts a unique tact here, showing love to be both an old and a new commandment. The idea that God’s people are to love one another is no new thing. From the earliest recorded history we find the saints of God attempting to walk together in harmony and fellowship, as for instance Abraham and his gracious dealings with Lot. Later, this would be codified in the Mosaic law. Love was first instituted as the duty of God’s people in that book most hated today by theological liberals who like to fancy they have discovered what genuine Christian love is. But God, speaking to Moses, commanded in Leviticus 19:18, “Thou shalt not avenge, nor bear any grudge against the children of thy people: I am the LORD.” He grounds their obedience to this command upon His own eternal power and Godhead. It is not a matter where the people of God have a choice. We are all to love our neighbor even as we love ourselves. James calls this the “royal law,” and Christ made it the foundation of His “golden rule.” Paul said that the whole law is fulfilled in this one word. Indeed, it is an old commandment, but a lasting commandment, and one of immeasurable value! This is the word which the saints had heard from the beginning. They knew, because it was inscribed in the law, endorsed by Christ, and expounded by the apostles, that they were to love their neighbors as themselves.
But John comes back again in verse 8 to say that he is writing a new commandment, a thing which is true in Christ and in His people. To what could this refer? The darkness, he says, is past, and the true light now shineth. To understand this, we must return to the upper room, where Christ had taken a towel, girded Himself, and proceeded to wash the feet of His disciples. The disciples were amazed at this humiliating behavior of their Lord, but He informed them that He had set them an example which they were to follow: “If I then, your Lord and Master, have washed your feet, ye also ought to wash one another’s feet.”