10 “Dearly beloved, I write unto you no new commandment, but the old commandment which you had from the beginning.” What commandment calls he “old? Which you had,” says he, “from the beginning. Old” then, in this regard, that you have already heard it: otherwise he will contradict the Lord, where He says, “A new commandment give I unto you, that you love one another.” But why an “old” commandment? Not as pertaining to the old man. But why? “Which you had from the beginning.
The old commandment is the word which you have heard.” Old then, in this regard, that you have already heard it. And the selfsame he shows to be new, saying, “Again, a new commandment write I unto you.” Not another, but the selfsame which he has called old, the same is also new. Why? “Which thing is true in Him and in you.” Why old, you have already heard: i.e., because you knew it already. But why new? “Because the darkness is past, and the true light now shines.” Lo, whence it is new: because the darkness pertains to the old man, but the light to the new man. What says the Apostle Paul? “Put off the old man, and put on the new.” And again what says he? “You were sometime darkness, but now light in the Lord.”
Source: Homilies on the First Epistle of John (New Advent)