You have just heard, beloved, the Lord saying to His disciples, “I have said these things to you so that my joy might be in you and your joy might be fulfilled” (Jn 15:11). What else is Christ's joy in us if not that He is pleased to rejoice over us? And what else is this joy of ours which He says is to be made full if not our having fellowship with Him? That is why He had said to blessed Peter, “If I do not wash you, you shall have no part with me” Jn 13:8). His joy in us, then, is the grace He has bestowed on us: and that is also our joy. He was rejoicing over it even from eternity, when He chose us before the foundation of the world (Eph 1:4). We cannot rightly say, however, that His joy was not full, for there was never a time when God's joy was imperfect. But that joy of His was not then in us: for we, in whom it could be, did not yet exist, and even when we began to exist, we did not begin to exist with that joy. In Him it always existed, however, for in the infallible truth of His foreknowledge He was rejoicing that one day we were to be His own. And so His joy over us was already full when He was rejoicing in foreknowing and foreordaining us... Nor, when He began to do what He foreknew that He would do, did that joy increase by which he is blessed, for that would mean that he became more blessed because he made us. Away with such a thought, brothers and sisters: God’s blessedness was not less without us and does not become greater because of us. His joy over our salvation, which always existed in Him when He foreknew and foreordained us, began to exist in us when He called us; and this joy we properly call our own since by it we, too, shall be blessed. But this joy of ours does increase and advance, and presses onward perseveringly to its own completion. It has its beginning in the faith of those reborn and its completion in the reward when they rise again.
This is how I understand the words, “I have said these things to you so that my joy might be in you, and that your joy might be fulfilled”: that my joy might be in you, that your joy might be made full. For my joy was always full, even before you were called, when ye were foreknown as those whom I was afterwards to call; but it comes to exist in you also, when you are made what I have foreknown you to be. And “that your joy might be fulfilled”: for you shall be blessed, which you are not as yet; just as you have now been created, who had no existence before. (In Ioannem Tr. 83, 1; PL 35, 1843-1844)