Hilary of Poitiers — as excerpted in the Catena Aurea on John 17:20-23

“(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.”