6 “And I have made known to them,” He says, “Your name, and will make it known.” I have made it known by faith, I will make it known by sight: I have made it known to those whose present sojourn in a strange land has its appointed end, I will make it known to those whose reign as kings shall be endless. “That the love,” He adds, wherewith [literally, which] You have loved me, may be in them, and I in them. (The form of speech is unusual, “the love, which You have loved me, may be in them, and I in them;” for the common way of speaking is, the love wherewith you have loved me.
Here, of course, it is a translation from the Greek: but there are similar forms also in Latin; as we say, He served a faithful service, He served as a soldier a strenuous soldier-service; when apparently we ought to have said, He served with a faithful service, he served as a soldier with a strenuous soldier-service. But such as the form of expression is, “the love which You have loved me;” one similar to it is also used by the apostle, “I have fought a good fight;” he does not say, in a good fight, which would be the more usual and perhaps correcter form of expression.)
But how else is the love wherewith the Father loved the Son in us also, but because we are His members and are loved in Him, since He is loved in the totality of His person, as both Head and members? Therefore He added, “and I in them;” as if saying, Since I am also in them. For in one sense He is in us as in His temple; but in another, because we are also Himself, seeing that, in accordance with His becoming man, that He might be our Head, we are His body. The Saviour's prayer is finished, His passion begins; let us, therefore, also finish the present discourse, that we may treat of His passion, as He grants us grace, in others to follow.
Source: Tractates on the Gospel of John (New Advent)