Hilary of Poitiers
Catena Aurea · Johannesevangelium 14
Philip saith unto him, Lord, shew us the Father, and it sufficeth us. Jesus saith unto him, Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thou then, Shew us the Father? Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works. Believe me that I am in the Father, and the Father in me: or else believe me for the very works’ sake.
(v. de Trin) The unchangeable God follows, so to speak, His own nature, by begetting unchangeable God. Nor does the perfect birth of unchangeable God from unchangeable God forsake His own nature. We understand then here the nature of God subsisting in Him, since God is in God, nor besides Him who is God, can any other be God.