John 6:44-51
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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Short on time here, all I can do is pass this on. Here's what Aquinas says. This is an intense read, but it's worth it.
Aquinas commentary on John 6: http://www.op-stjoseph.org/Students/study/thomas/John6.htm
957 He said, I am the living bread; consequently, I can give life. Material bread does not give life forever, because it does not have life in itself; but it gives life by being changed and converted into nourishment by the energy of a living organism. That has come down from heaven: it was explained before [4671 how the Word came down. This refuted those heresies which taught that Christ was a mere man, because according to them, he would not have come down from heaven.
958 He has the power to give eternal life; thus he says, If anyone eats of this bread, i.e., spiritually, he will live, not only in the present through faith and justice, but forever. “Everyone who lives and believes in me, will never die” (below 11:26).
959 He then speaks of his body when he says, And the bread which I will give is my flesh. For he had said that he was the living bread; and so that we do not think that he is such so far as he is the Word or in his soul alone, he shows that even his flesh is life-giving, for it is an instrument of his divinity. Thus, since an instrument acts by virtue of the agent, then just as the divinity of Christ is lifegiving, so too his flesh gives life (as Damascene says) because of the Word to which it is united. Thus Christ healed the sick by his touch. So what he said above, I am the living bread, pertained to the power of the Word; but what he is saying here pertains to the sharing in his body, that is, to the sacrament of the Eucharist.
960 We can consider four things about this sacrament: its species, the authority of the one who instituted it, the truth of this sacrament, and its usefulness.As to the species of this sacrament: This is the bread; “Come, and eat my bread” (Prv 9:5). The reason for this is that this is the sacrament of the body of Christ; but the body of Christ is the Church, which arises out of many believers forming a bodily unity: “We are one body” (Rom 12:5). And so because bread is formed from many grains, it is a fitting species for this sacrament. Hence he says, And the bread which I will give is my flesh.
961 The author of this sacrament is Christ: for although the priest confers it, it is Christ himself who gives the power to this sacrament, because the priest consecrates in the person of Christ. Thus in the other sacraments the priest uses his own words or those of the Church, but in this sacrament he uses the words of Christ: because just as Christ gave his body to death by his own will, so it is by his own power that he gives himself as food: “Jesus took bread, he blessed it and broke it, and gave it to his disciples, saying: ‘Take and eat it, this is my body’ “ (Mt 26:26). Thus he says, which I will give; and he says, will give, because this sacrament had not yet been instituted.
962 The truth of this sacrament is indicated when he says, is my flesh. He does not say, “This signifies my flesh,” but it is my flesh, for in reality that which is taken is truly the body of Christ: “Who will give us his flesh so that we may be satisfied?” as we read in Job (31:3 1).Since the whole Christ is contained in this sacrament, why did he just say, this is my flesh? To answer this, we should note that in this mystical sacrament the whole Christ is really contained: but his body is there by virtue of the conversion; while his soul and divinity are present by natural concomitance. For if we were to suppose whal is really inipossible, that is, that the divinity of Christ is separated from his body, then his divinity would not be present in this sacrament. Similarly, if someone had consecrated during the three days Christ was dead, his soul would not have been present there [in the sacrament], but his body would have been, as it was on the cross or in the tomb. Since this sacrament is the commemoration of our Lord’s passion—according to “As often as you eat this bread and drink this cup, you proclaim the death of the Lord” (1 Cor 11:26)—and the passion of Christ depended on his weakness—according to “He was crucified through weakness” (2 Cor 13:4)—he rather says, is my flesh, to suggest the weakness through which he died, for “flesh” signifies weakness.
963 The usefulness of this sacrament is great and universal. It is great, indeed, because it produces spiritual life within us now, and will later produce eternal life, as was said. For as is clear from what was said, since this is the sacrament of our Lord’s passion, it contains in itself the Christ who suffered. Thus, whatever is an effect of our Lord’s passion is also an effect of this sacrament. For this sacrament is nothing other than the application of our Lord’s passion to us. For it was not fitting for Christ to be always with us in his own presence; and so he wanted to make up for this absence through this sacrament. Hence it is clear that the destruction of death, which Christ accomplished by his death, and the restoration of life, which he accomplished by his resurrection, are effects of this sacrament.
964 The usefulness of this sacrament is universal because the life it gives is not only the life of one person, but, so far as concerns itself, the life of the entire world: and for this the death of Christ is fully sufficient. “He is the offering for our sins; and not for ours only, but also for those of the entire world” (1 Jn 2:2).We should note that this sacrament is different from the others: for the other sacraments have individual effects: as in baptism, only the one baptized receives grace. But in the immolation of this sacrament, the effect is universal: because it affects not just the priest, but also those for whom he prays, as well as the entire Church, of the living and of the dead. The reason for this is that it contains the universal cause of all the sacraments, Christ. Nevertheless, when a lay person receives this sacrament it does not benefit others ex opere operato [by its own power] considered as a receiving. However, due to the intention of the person who is acting and receiving, it can be communicated to all those to whom he directs his intention. It is clear from this that lay persons are mistaken when they receive the Eucharist for those in purgatory.
