Albert Barnes Commentary 1 John 2:8

"Again, a new commandment write I unto you, which thing is true in him and in you; because the darkness is passing away, and the true light already shineth." — 1 John 2:8 (ASV)

Again, a new commandment I write to you. "And yet, that which I write to you, and particularly command you, deserves in another sense to be called a new commandment, though it has also been taught from the beginning, for it was called new by the Savior himself." Or the meaning may be, "In addition to the general precepts to which I have referred, I now call your attention to the new commandment of the Savior, that which he himself called new." There can be no doubt here that John refers to the commandment to love one another (1 John 2:9–11), and that it is here called new, not in the sense that John taught it as a novel doctrine, but in the sense that the Savior called it such. For the reasons why it was so called by him, see the notes on 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 interaction with his disciples. That which was most prominent in him was this very love which he commanded all his followers.

And in you. Among you. That is, you have demonstrated it in your interaction with each other. It is not new in the sense that you have never heard of it and have never demonstrated it, but only in the sense that he called it new.

Because the darkness is past, and the true light now shines. 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 on John 1:4 and John 1:5.

Compare John 8:12; John 12:35, 36, 46; Isaiah 9:2.