Augustine of Hippo
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.
(Tr. cx) We must particularly observe here, that our Lord did not say, that we may be all one, but that they may be all one, as Thou, Father, in Me, and I in Thee, are one, understood. For the Father is so in the Son, that They are one, because They are of one substance; but we can be one in Them, but not with Them; because we and They are not of one substance. They are in us, and we in Them, so as that They are one in Their nature, we one in ours. They are in us, as God is in the temple; we in Them, as the creatureis in its Creator. Wherefore He adds, in Us, to shew, that our being made one by charity, is to be attributed to the grace of God, not to ourselves.