5 In close relation to these come also His further words: “And You have loved them as you have loved me.” That is to say, in the Son the Father loves us, because in Him He has chosen us before the foundation of the world. For He who loves the Only-begotten, certainly loves also His members which, through His in strumentality, He engrafted into Him by adoption. But we are not on this account equal to the only-begotten Son, by whom we have been created and re-created, that it is said, “You have loved them as [You have] also [loved] me.”
For one does not always intimate equality when he says, As this, so also that other; but sometimes only, Because this is, so also is the other; or, That the one is, in order that the other may be also. For who could say that the apostles were sent by Christ into the world in exactly the same way as He Himself was sent by the Father? For, to say nothing of other differences, which it would be tedious to mention, they at all events were sent when they were already men; but He was sent in order that He might be man; and yet He said above, “As You have sent me into the world, even so have I sent them into the world;” as if He had said, Because You have sent me, I have sent them.
So also in the passage before us He says, “You have loved them, as You have loved me;” which is nothing else than this, You have loved them because that You have also loved me. For He could not but love the members of the Son, seeing that He loves the Son Himself; nor is there any other reason for loving His members, save that He loves Himself. But He loves the Son as regards His Godhead, because He begot Him equal with Himself; He loves Him also in regard to what He is as man, because the Only-begotten Word was Himself made flesh, and on account of the Word is the flesh of the Word dear to Him; but He loves us, inasmuch as we are the members of Him whom He loves; and in order that we might be so, He loved us on this account before we existed.
Source: Tractates on the Gospel of John (New Advent)