Augustine of Hippo
Catena Aurea · Johannesevangelium 17
I pray for them: I pray not for the world, but for them which thou hast given me; for they are thine. And all mine are thine, and thine are mine; and I am glorified in them. And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are. While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled. And now come I to thee; and these things I speak in the world, that they might have my joy fulfilled in themselves.
(iv. de Trin. c. ix) He does not say, That I and they maybe one, though He might have said so in the sense, that He was the head of the Church, and the Church His body; not one thing, but one person: the head and the body being one Christ. But shewing something else, viz. that His divinity is consubstantial with the Father, He prays that His people may in like manner be one; but one in Christ, not only by the same nature, in which mortal man is made equal to the Angels, but also by the same will, agreeing most entirely in the same mind, and melted into one Spirit by the fire of love. This is the meaning of, That they may be one as We are: viz. that as the Father and the Son are one not only by equality of substance, but also in will, so they, between whom and God the Son is Mediator, may be one not only by the union of nature, but by the union of love.