5 For I think that His words, “But I will see you again, and your heart shall rejoice, and your joy no man takes from you,” are not to be referred to the time of His resurrection, and when He showed them His flesh to be looked at and handled; but rather to that of which He had already said, “He that loves me, shall be loved of my Father; and I will love him, and will manifest myself to him.” For He had already risen, He had already shown Himself to them in the flesh, and He was already sitting at the right hand of the Father, when that same Apostle John, whose Gospel this is, says in his epistle, “Beloved, now are we the sons of God; and it does not yet appear what we shall be: but we know that, when He shall be manifested, we shall be like Him; for we shall see Him as He is.” That vision belongs not to this life, but to the future; and is not temporal, but eternal.
“And this is life eternal,” in the words of Him who is that life, “that they might know You the only true God, and Jesus Christ, whom You have sent.” Of this vision and knowledge the apostle says, “Now we see through a glass, in a riddle; but then face to face: now I know in part; but then shall I know even as also I am known.” At present the Church is in travail with the longing for this fruit of all her labor, but then she shall bring to the birth in its actual contemplation; now she travails in birth with groaning, then shall she bring forth in joy; now she travails in birth through her prayers, then shall she bring forth in her praises.
Thus, too, is it a male child; since to such fruit in the contemplation are all the duties of her present conduct to be referred. For He alone is free; because He is desired on His own account, and not in reference to anything besides. Such conduct is in His service; for whatever is done in a good spirit has a reference to Him, because it is done on His behalf; while He, on the other hand, is got and held in possession on His own account, and not on that of anything besides. And there, accordingly, we find the only end that is satisfying to ourselves.
He will therefore be eternal; for no end can satisfy us, save that which is found in Him who is endless. With this was Philip inspired, when he said, “Show us the Father, and it suffices us.” And in that showing the Son gave promise also of His own presence, when He said, “Do you not believe that I am in the Father, and the Father in me?” Of that, therefore, which alone suffices us, we are very appropriately informed, “Your joy no man takes from you.”
Source: Tractates on the Gospel of John (New Advent)