The text below is taken from the Treatise on the Trinity by St. Hilary of Poitiers (315-368, AD) and discusses the unity of the faithful in God through the incarnation of the Word and the sacrament of the Eucharist.
If the Word has truly been made flesh and we in very truth receive the Word made flesh as food from the Lord, are we not bound to believe that he abides in us naturally? Born as a man, he assumed the nature of our flesh so that now it is inseparable from himself, and conjoined the nature of his own flesh to the nature of the eternal Godhead in the sacrament by which his flesh is communicated to us. Accordingly we are all one, because the Father is in Christ and Christ in us. He himself is in us through the flesh and we in him, and because we are united with him, our own being is in God.
He himself testifies that we are in him through the sacrament of the flesh and blood bestowed upon us: In a short time the world will no longer see me; but you will see me, because I live and you will live. On that day you will understand that I am in my Father and you in me and I in you. If he wanted to indicate a mere unity of will, why did he set forth a kind of gradation and sequence in the completion of that unity? It can only be that, since he was in the Father through the nature of Deity, and we on the contrary in him through his birth in the body, he wishes us to believe that he is in us through the mystery of the sacraments. From this we can learn the perfect unity through a Mediator; for we abide in him and he abides in the Father, and while abiding in the Father he abides in us as well – so that we attain unity with the Father. For while Christ is in the Father naturally according to his birth, we too are in Christ naturally, since he abides in us naturally.
He himself has told us how natural this unity is: He who eats my flesh and drinks my blood lives in me and I live in him. No-one can be in Christ unless Christ is in him, because the only flesh which he has taken to himself is the flesh of those who have taken his.
He had earlier revealed to us the sacrament of this perfect unity: As I, who am sent by the living Father, myself draw life from the Father, so whoever eats me will draw life from me. He lives because of the Father, and as he lives because of the Father so we live because of his flesh.
Every comparison is chosen to shape our understanding, so that we may grasp the subject concerned by help of the analogy set before us. To summarise, this is what gives us life: that we have Christ dwelling within our carnal selves through the flesh, and we shall live because of him in the same manner as he lives because of the Father.