Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth. Which thing is true in him and in you - It is true that Christ loved the world so well as to lay down his life for it; and it was true in them, in all his faithful followers at that time, who were ready to lay down their lives for the testimony of Jesus. There is a saying in Synopsis Sohar, p. 94, n. 51, that may cast some light on this passage: That way in which the just have walked, although it be Old, yet may be said to be New in the love of the righteous. The love that the righteous bear to God and to each other is a renewal of the commandment.
The darkness is past - The total thick darkness of the heathen world, and the comparative darkness of the Mosaic dispensation, are now passing away; and the pure and superior light of Christianity is now diffusing its beams everywhere. He does not say that the darkness was all gone by, but παραγεται, it is passing away; he does not say that the fullness of the light had appeared, but ηδη φαινει, it is now shining and will shine more and more to the perfect day; for the darkness passes away in proportion as the light shines and increases.