51 I am the living bread which came down from heaven. If any man eat of this bread, he shall live for ever; and the bread that I will give, is my flesh, for the life of the world.
Augustine: The manna too came down from heaven; but the manna was shadow, this is substance.
Chrysostom: Their fathers eat manna and were dead; whereas of this bread He says, that a man may eat thereof, and not die. The difference of the two is evident from the difference of their ends.
Augustine: Our Lord pronounces Himself to be bread, not only in respect of that Divinity, which feeds all things, but also in respect of that human nature, which was assumed by the Word of God.
Bede: This bread our Lord then gave, when He delivered to His disciple the mystery of His Body and Blood, and offered Himself to God the Father on the altar of the cross. For the life of the world, i.e. not for the elements, but for mankind, who are called the world.
Theophylact: Which I shall give: this shows His power; for it shows 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. . . . 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.
Augustine: When does flesh receive the bread which He calls His flesh? The faithful know and receive the Body of Christ, if they labor to be the body of Christ. And they become the body of Christ, if they study to live by the Spirit of Christ: for that which lives by the Spirit of Christ, is the body of Christ. This bread the Apostle sets forth, where he says, We being many are one body [1 Cor 12:12].
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. IV (London: Rivington, 1845).