2 Whosoever then you are that hast your thoughts on Christ, and desirest to receive what He has promised, be not slow to do that which He has enjoined. Now what has He promised? “Eternal life.” And what has He enjoined? That pardon be given to your brother. As if He had said to you, “O man, give pardon to a man, so that I, who am God, may come unto you.” But that I may pass over, or rather pass by for a while, those more exalted divine promises in which our Creator engages to make us equal with His Angels, that we may with Him, and in Him, and by Him, live without end; not to speak of this just now, do you not wish to receive of your God this very thing, which you are commanded to give your brother? This very thing, I say, which you are commanded to give your brother, do you not wish to receive from your Lord? Tell me if you wish it not; and so give it not. What is this, but that you should forgive him that asks you, if you require to be forgiven? But if you have nothing to be forgiven you, I dare to say, be unwilling to forgive. Though I ought not even to say this. Though you have nothing to be forgiven you, forgive.
Source: Sermons on Selected Lessons of the New Testament (New Advent)