7 The Son therefore goes on to say: “Those that You gave me, I have kept, and none of them is lost, but the son of perdition; that the Scripture might be fulfilled.” The betrayer of Christ was called the son of perdition, as foreordained to perdition, according to the Scripture, where it is specially prophesied of him in the 109th Psalm.
8. “And now,” He says, “come I to You; and these things I speak in the world, that they may have my joy fulfilled in themselves.” See! He says that He speaks in the world, when He had said only a little before, “I am no more in the world:” the reason of which we have there explained, or rather have shown that He Himself explained it. Accordingly, on the one hand, as He had not yet departed, He was still here; and because He was on the very point of departure, in a kind of way He was no more here. But what this joy is whereof He says, “That they may have my joy fulfilled in themselves,” has already been elucidated above, where He says, “That they may be one, even as we are.” This joy of His that is bestowed on them by Him, was to be fulfilled, He says, in them; and for that very end declared that He had spoken in the world. This is that peace and blessedness in the world to come, for the attaining of which we must live temperately, and righteously, and godly in the present.
Source: Tractates on the Gospel of John (New Advent)