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.
(vii de Trin) And this unity is recommended by the great example of unity: As Thou, Father, art in Me, and I in Thee, that they also may be one in Us, i. e. that as the Father is in the Son, and the Son in the Father, so, after the likeness of this unity, all may be one in the Father and in the Son.