Augustine of Hippo — as excerpted in the Catena Aurea on John 17:20-23

“(Tr. cx. 4) Nor is this said, however, as if to mean that the Father was not in us, or we in the Father. He only means to say, that He is Mediator between God and man. And what He adds, That they may be made perfect in one, shews that the reconciliation made by this Mediator, was carried on even to the enjoyment of everlasting blessedness. So what follows, That the world may know that Thou hast sent Me, must not be taken to mean the same as the words just above, That the world may believe. For as long as we believe what we do not see, we are not yet made perfect, as we shall be when we have merited to see what we believe. So that when He speaks of their being made perfect, we are to understand such a knowledge as shall be by sight, not such as is by faith. These that believe are the world, not a permanent enemy, but changed from an enemy to a friend; as it follows: And hast loved them, as Thou hast loved Me. The Father loves us in the Son, because He elected us in Him. These words do not prove that we are equal to the Only Begotten Son; for this mode of expression, as one thing so another, does not always signify equality. It sometimes only means, because one thing, therefore another. And this is its meaning here: Thou hast loved them, as Thou hast loved Me, i. e. Thou hast loved them, because Thou hast loved Me. There is no reason for God loving His members, but that He loves him. But since He hateth nothing that He hath made, who can adequately express how much He loves the members of His Only Begotten Son, and still more the Only Begotten Himself.”