How can they say that the flesh, which is nourished with the body of the Lord and with His blood, goes to corruption, and does not partake of life? Let them, therefore, either alter their opinion, or cease from offering the things just mentioned. But our opinion is in accordance with the Eucharist, and the Eucharist in turn establishes our opinion. For we offer to Him His own, announcing consistently the fellowship and union of the flesh and Spirit. For as the bread, which is produced from the earth, when it receives the invocation of God, is no longer common bread, but the Eucharist, consisting of two realities, earthly and heavenly; so also our bodies, when they receive the Eucharist, are no longer corruptible, having the hope of the resurrection to eternity…
Vain above all are they who despise the whole dispensation of God, and deny the salvation of the flesh and reject its rebirth, saying that it is not capable of incorruption.
For if our mortal flesh is not saved, then neither did the Lord redeem us by his blood, nor is the cup of the Eucharist the communion of his blood, and the bread which we break the communion of his body.
For blood is only to be found in veins and flesh, and the rest of human nature, of which the Word of God was indeed made partaker; it was with his own blood that he redeemed us. So also his apostle says, “In whom we have redemption by his blood, and the remission of sins.”
For since we are his members, and are nourished by his creation – for it is he who causes the sun to rise and the rain to fall – he declares that the cup, which comes from his creation, is his own blood, and he makes it the nourishment of our blood. He assures us that the bread, which comes from his creation, is his body, and he makes it the nourishment of our body.
For when the mixed cup and the baked bread receive the word of God, and become the Eucharist, the body and blood of Christ, and by these our flesh grows and is confirmed – how can they say that flesh cannot receive the free gift of God, which is eternal life, since it is nourished by the body and blood of the Lord, and made a member of him?
As the blessed Paul says in the Epistle to the Ephesians that we are members of his body, of his flesh and his bones. He does not say this about a merely spiritual and invisible man, for spirits do not have flesh and bones, but about that dispensation by which the Lord became a real man, consisting of flesh, and nerves, and bones: That flesh which is nourished by the cup which is his blood, and grows by the bread which is his body.
A cutting from a vine planted in the ground bears fruit in its own time. A grain of wheat, falling into the ground and there dissolving, rises with great increase by the Spirit of God who sustains all things, The wisdom of God places these things at the service of man and when they receive God’s word they become the Eucharist, which is the body and blood of Christ.
In the same way also our bodies which are nourished by it, and then fall into the earth and are dissolved therein, will rise at the proper time, for the Word of God will raise them up to the glory of God the Father.
It is he who freely grants to this mortal immortality, and gives to the corruptible the gracious gift of incorruption, for the strength of God is made perfect in weakness.
(From Against Heresies 4:18:5; 5:2:2-3)