St. Hilary: On the Trinity, Book VIII: 14-16

The words in which we speak of the things of God must be used in no mere human and worldly sense, nor must the perverseness of an alien and impious interpretation be extorted from the soundness of heavenly words by any violent and headstrong preaching. Let us read what is written, let us understand what we read, and then fulfil the demands of a perfect faith. For as to what we say concerning the reality of Christ's nature within us, unless we have been taught by Him, our words are foolish and impious. For He says Himself, My flesh is meat indeed, and My blood is drink indeed. He that eats My flesh and drinks My blood abides in Me, and I in him.  As to the verity of the flesh and blood there is no room left for doubt. For now both from the declaration of the Lord Himself and our own faith, it is verily flesh and verily blood. And these when eaten and drunk, bring it to pass that both we are in Christ and Christ in us. Is not this true? Yet they who affirm that Christ Jesus is not truly God are welcome to find it false. He therefore Himself is in us through the flesh and we in Him, while together with Him our own selves are in God.

Now how it is that we are in Him through the sacrament of the flesh and blood bestowed upon us, He Himself testifies, saying, And 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 did He set forth a kind of gradation and sequence in the completion of the unity, unless it were 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 would have us believe that He is in us through the mystery of the sacraments? And thus there might be taught a perfect unity through a Mediator, while, we abiding in Him, He abode in the Father, and as abiding in the Father abode also in us; and so we might arrive at unity with the Father, since in Him Who dwells naturally in the Father by birth, we also dwell naturally, while He Himself abides naturally in us also.

Again, how natural this unity is in us He has Himself testified on this wise — He who eats My flesh and drinks My blood abides in Me, and I in him.  For no man shall dwell in Him, save him in whom He dwells Himself, for the only flesh which He has taken to Himself is the flesh of those who have taken His. Now He had already taught before the sacrament of this perfect unity, saying, As the living Father sent Me, and I live through the Father, so he that eats My flesh shall himself also live through Me. So then He lives through the Father, and as He lives through the Father in like manner we live through His flesh.