John 6:41-51
41 The Jews murmured about him because he said, "I am the bread that came down from heaven,"
42 and they said, "Is this not Jesus, the son of Joseph? Do we not know his father and mother? Then how can he say, 'I have come down from heaven'?"
43 Jesus answered and said to them, "Stop murmuring among yourselves.
44 No one can come to me unless the Father who sent me draw him, and I will raise him on the last day.
45 It is written in the prophets: 'They shall all be taught by God.' Everyone who listens to my Father and learns from him comes to me.
46 Not that anyone has seen the Father except the one who is from God; he has seen the Father.
47 Amen, amen, I say to you, whoever believes has eternal life.
48 I am the bread of life.
49 Your ancestors ate the manna in the desert, but they died;
50 this is the bread that comes down from heaven so that one may eat it and not die.
51 I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world."
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Since the earliest times, the Church has understood this as a direct reference to the Eucharist. From the Catena Aurea:
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: And the bread, He says, that I will give is My flesh, which I will give for the life of the world.
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.
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.
AUGUSTINE. But 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. O sacrament of mercy, O sign of unity, O bond of love! Whoso wishes to live, let him draw nigh, believe, be incorporated, that he may be quickened.
