Theophylact of Ohrid — as excerpted in the Catena Aurea on John 6:51

“Which I shall give: this shews His power; for it shews that He was not crucified as a servant, in subjection to the Father, but of his own accord; for though He is said to have been given up by the Father, yet He delivered Himself up also. And observe, the bread which is taken by us in the mysteries, is not only the sign of Christ’s flesh, but is itself the very flesh of Christ; for He does not say, The bread which I will give, is the sign of My flesh, but, is My flesh. The bread is by a mystical benediction conveyed in unutterable words, and by the indwelling of the Holy Ghost, transmuted into the flesh of Christ. But why see we not the flesh? Because, if the flesh were seen, it would revolt us to such a degree, that we should be unable to partake of it. And therefore in condescension to our infirmity, the mystical food is given to us under an appearance suitable to our minds. He gave His flesh for the life of the world, in that, by dying, He destroyed death. By the life of the world too, I understand the resurrection; our Lord’s death having brought about the resurrection of the whole human race. It may mean too the sanctified, beatified, spiritual life; for though all have not attained to this life, yet our Lord gave Himself for the world, and, as far as lies in Him, the whole world is sanctified.”