Chapter I.
Since the true flesh and true blood of Christ are in the sacrament, consecrated by the divine power of the Word, and veiled under a different appearance so as not to cause offense, from this we understand that we participate in the divine substance in that food.
Just as Jesus Christ, our Lord, is true God, not in the same way as men through grace, but as the Son from the substance of the Father; so true is his flesh, as he himself said, which we receive, and true is his blood.
But perhaps you might say (what the disciples of Christ themselves said at that time, hearing him say, 'Unless you eat the flesh of the Son of Man and drink his blood, you have no life in you' - John 6:54): Perhaps you might say, How can this be true? I see a likeness, but I do not see the truth of blood.
First of all I have told you (Sup. lib. IV, cap. 4) about the discourse of Christ, which works so that it can change and convert the established forms of nature (John VI, 67). Then when the disciples heard that Christ would give them His flesh to eat and His blood to drink, they withdrew; only Peter said: Do you have the words of eternal life, and how can I leave you? (Ibid., 69). So, in order not to have many say this, as if there were a horror of blood, but for the grace of redemption to remain, you take the sacrament in a likeness, but truly you attain the grace and power of nature.
'I am,' he says, 'the living bread which came down from heaven.' But flesh did not descend from heaven, that is, He assumed flesh on earth from the Virgin. How, then, did the bread descend from heaven, and the living bread? Because our Lord Jesus Christ is the same, both a sharer in divinity and in body: and you who receive the flesh participate in that divine substance as nourishment.