Our apostle here exhorts Christians to the great duty of brotherly love, assuring them, that it was no new commandment which he enjoined them, but that which they were taught, not only in and by the Old Testament, but at the first preaching of the gospel amongst them; and in these respects the command of love might be called an old commandment, it being a branch of the law of nature, and a known precept of the Jewish religion; although in other respects it might be called a new commandment, because urged from a new motive, and enforced by a new example.
Learn hence, 1. That the doctrine of Christian love is a divine commandment , that which Christians are not only allowed, but enjoined to practice; and it is called the commandment, in the singular number, to intimate, that in this one commandment all the rest were contained, so that in keeping this we keep all. What are all the commandments indeed but love enlarged? And what is love but the commandments contracted?
Learn, 2. That the commandment of love is an old commandment; it is as old as Moses, yea, as old as Adam; being a part of the law of nature written in Adam's heart. The evangelical command of love was from the beginning of the law, and nothing new enjoined by Christ, which was not before by Moses.
Learn, 3. That yet this command of love may, in some respect, be called a new commandment; not substantially, but circumstantially: not in the essence of the doctrine, but in the manner of the discovery; not in respect of the truth delivered, but in the way of delivering. New, not in regard of institution, but restitution, because purged from the old corrupt glossed of the Pharisees, who had limited this duty of love, and confined it to their our countrymen; whereas Christ obliges his disciples to love mankind, even our very enemies. In a word, it may be called a new commandment, because it was never to wax old, but to be always fresh in the memory and practice of Christ's disciples to the end of the world.
Observe next, The arguments to enforce the observation of this new commandment;
1. In those word, which thing is true in him and in you: that is, as there was in Christ a true and sincere love towards you, so look that there be a true and sincere love in you towards him, and one towards another.
2. Because the darkness is past, and the true light now shineth; that is, the darkness of ignorance in general, and the darkness of the Mosaic state in particular, is now past, and the true light of the gospel now clearly shineth.