Flesh for the Life of the World

In today's Gospel, Jesus speaks of "the bread which I shall give": this shows His power; for it shows that He was not crucified as a servant, subject 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 willingly. Observe, the bread which is taken by us in the sacraments, is not only the sign of Christ's flesh, but is itself the very flesh of Christ; for He does not say, "is the sign of My flesh," but, "is My flesh." The bread is by a mystical benediction and by the indwelling of the Holy Spirit, transmuted into the flesh of Christ.

But why do we not see flesh? Because, if 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 human weakness, the mystical food is given to us under an appearance suitable to our minds. He gave His flesh for the life of the world, that by dying He destroyed death. By the life of the world too, we understand the resurrection; our Lord's death having brought about the resurrection of the whole human race.

參考 References:
Catena Aurea (St. Thomas Aquinas)
Explanations on the Gospels (St. Theophylact of Ochrid)