From On the Trinity, VIII, 13-16 (ca. 360)
We believe the Word became flesh and that we receive the Word made flesh as food from the Lord. How then can we fail to believe that he really dwells within us? When he became human, he assumed the nature of our flesh, uniting it himself forever. In the sacrament of his body, he actually gives us his own flesh, which he has united to his divinity. Thus we are all one, because the Father is in Christ and Christ in us. He is in us through his flesh, and we are in him. With him we form a unity, which is in God. …
The manner of our indwelling in Christ through the sacrament of his own body and blood is evident from the Lord’s own words, “The world will no longer see me, but you shall see me; because I live you shall live also; because I am in My Father, and you in me, and I in you.
If he wished to indicate a mere unity of will, why should he have given us this explanation of the steps by which it is achieved? He is in the Father by reason of his divine nature, and we are in him by reason of his human birth, and he is in us through the mystery of the sacraments.
This, surely, is what he wished us to believe; this is how he wanted us to understand the perfect unity that is achieved through our mediator, who lives in the Father while we live in him, and who, while living on the Father, lives also in us. This is how we attain to unity with the Father. Christ is in very truth in the Father by his eternal generation; we are in very truth in Christ, and he likewise is in us….
We draw life from his flesh just as he draws life from the Father. Such comparisons aid our understanding, since we can grasp a point more easily when we have an analogy. And the point is that Christ is the wellspring of our life. Since we who are in the flesh have Christ dwelling in us through his flesh, we shall draw life from him in the same way as he draws life from the Father.
St. Hilary of Poitiers (ca. 310-ca.367) was Bishop of Poitiers and an influential theologian, who wrote important works defending the Christological doctrine of the Council of Nicaea against its Arian opponents, especially On the Trinity, a work of his final period. He was exiled for his faithfulness to orthodox teaching. Hilary’s feast day is January 13.