Hilary of Poitiers
Catena Aurea · Johannesevangelium 17
Neither pray I for these alone, but for them also which shall believe on me through their word; That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me. And the glory which thou gavest me I have given them; that they may be one, even as we are one: I in them, and thou in me, that they may be made perfect in one: and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.
(viii. de Trin) By this giving and receiving of honour, then, all are one. But I do not yet apprehend in what way this makes all one. Our Lord, however, explains the gradation and order in the consummating of this unity, when He adds, I in them, and Thou in Me; so that inasmuch as He was in the Father by His divine nature, we in Him by His incarnation, and He again in us by the mystery of the sacrament, a perfect union by means of a Mediator was established.