1 John 2:8

Again, a new commandment; as much as to say, I have called it an old commandment; but there is a sense in which it is new.

Which thing is true; it is true that it is a new commandment.

In him and in you; in the case of Christ who has given it, and in the case of you who have received it.

Because the darkness is past; more literally, is passing away; he means the darkness that existed before the light of Christ's gospel was revealed.

The true light; which Christ has brought into the world. John 8:12; John 9:5; John 12:35-36. This light is all summed up in the great commandment of love, which Christ has given and his disciples have received as a new commandment, because it is exemplified by himself in a new way and enforced upon them by new motives. Compare, besides the texts quoted above from John, 1Jn 3:16; 1 John 4:9-11; Ephesians 4:32; Ephesians 5:2; Ephesians 5:25; Philippians 2:5-8.