Again, a new commandment I write unto you - “And yet, that which I write to you, and particularly enjoin on you, deserves in another sense to be called a new commandment, though it has been also inculcated from the beginning, for it was called new by the Saviour himself.” Or the meaning may be, “In addition to the general precepts which I have referred to, I do now call your attention to the new commandment of the Saviour, that which he himself called new.” There can be no doubt here that John refers to the commandment to “love one another,” (see 1 John 2:9-11), and that it is here called new, not in the sense that John inculcated it as a novel doctrine, but in the sense that the Saviour called it such. For the reasons why it was so called by him, see the notes at John 13:34.
Which thing is true in him - In the Lord Jesus. That is, which commandment or law of love was illustrated in him, or was manifested by him in his contact with his disciples. That which was most prominent in him was this very love which he enjoined on all his followers.
And in you - Among you. That is, you have manifested it in your contact with each other. It is not new in the sense that you have never heard of it, and have never evinced it, but in the sense only that he called it new.
Because the darkness is past, and the true light now shineth - The ancient systems of error, under which people hated each other, have passed away, and you are brought into the light of the true religion. Once you were in darkness, like others; now the light of the pure gospel shines around you, and that requires, as its distinguishing characteristic, love. Religion is often represented as light; and Christ spoke of himself, and was spoken of, as the Light of the world. See the notes at John 1:4-5. Compare John 8:12; John 12:35-36, John 12:46; Isaiah 9:2.