I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever. And the bread that I will give for the life of the world is my flesh.” John 6:51
Now if Christ’s flesh is distinguished from all flesh and is solely and pre-eminently a spiritual flesh, born not of the flesh but of the Spirit, ten it is also a spiritual food. If it is a spiritual food, it is an eternal food which cannot perish, as Christ himself says in John 6 [:27], “Labor for the food which does not perish, which the Son of man will give to you;” and again, “I am the living bread which came down from heaven” [6:51]; again, “If anyone eats of me, he will live for ever” [6:51]. And so, throughout the entire chapter he teaches that his flesh is the true, living, eternal food which gives life and sustains all who eat of it; and he who does not eat of it must die, etc.
Why? For this reason: His flesh is not of flesh, or fleshly, but spiritual; therefore it cannot be consumed, digested, and transformed, for it is imperishable as is all that is of the Spirit, and a food of an entirely different kind from perishable food. Perishable food is transformed into the body which eats it; this food, however, transforms the person who eats it into what it is itself, and makes him like itself, spiritual, alive, and eternal; as Christ says, “This is the bread from heaven, which gives life to the world” [6:33].
Whether Christ’s flesh is eaten physically or spiritually, then, it is the same body, the same spiritual flesh, the same imperishable food which in the Supper is eaten physically with the mouth and spiritually with the heart, according to Christ’s institution, or eaten spiritually with the heart alone through the Word, as he teaches in John 6 [:63]. For the fact that it is eaten physically with the mouth in the Supper does not prevent it at all from becoming flesh or a fleshly food. On the contrary, whether it enters the mouth or the heart, it is the same body; just as when he walked on earth, he remained the same Christ, whether he came into the hands of the faithful or of the wicked.
That These Words of Christ, “This is My Body,” etc., Still Stand Firm against the Fanatics 1527
Luther’s Works volume 37: Word and Sacrament III 99-100