A bread already filled with the yeast of the earth leaves no room for any other leaven. Which is to say that as far as "a sign" goes—and the Eucharist is far more than a mere sign!—the unleavened host is a waiting vessel into which the Lord of Heaven and Earth, the Resurrected Christ, "pours" Himself and invites us to communion with Him. We, too, must become unleavened so that Jesus, finding a welcoming home, may empty Himself into us.
Are we not called to become leaven in the world for the salvation of souls? A soul filled with Jesus recognizes Jesus hidden in others. Recall how Saint Teresa of Calcutta served Jesus (hidden) among the poorest of the poor. The impoverished person, that is to say the humble person freed by grace, leaves much room for Jesus to dwell under his or her unworthy roof.
Jesus first repeated what he said, then summarized: "‘I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh.’ The Jews then disputed among themselves, saying, ‘How can this man give us his flesh to eat?’" (John 6:51–52).
His listeners were stupefied because now they understood Jesus literally—and correctly. He again repeated his words, but with even greater emphasis, and introduced the statement about drinking his blood: "Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him" (John 6:53–56).
Notice that Jesus made no attempt to soften what he said, no attempt to correct "misunderstandings," for there were none. Our Lord’s listeners understood him perfectly well. They no longer thought he was speaking metaphorically. If they had, if they mistook what he said, why no correction?The Holy Spirit, Who communicates the Presence of Jesus, fuels man's hunger for the Bread of Heaven and causes him to rise up to meet his God and Saviour. The Holy Spirit awakens the soul to the Son of God Who offers himself in and through the consecrated host (Body of Christ) and consecrated wine (Blood of Christ).
On other occasions when there was confusion, Christ explained just what he meant (cf. Matt. 16:5–12). Here, where any misunderstanding would be fatal, there was no effort by Jesus to correct. Instead, he repeated himself for greater emphasis.
In John 6:60 we read: "Many of his disciples, when they heard it, said, ‘This is a hard saying; who can listen to it?’" These were his disciples, people used to his remarkable ways. He warned them not to think carnally, but spiritually: "It is the Spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life" (John 6:63; cf. 1 Cor. 2:12–14).
But he knew some did not believe. (It is here, in the rejection of the Eucharist, that Judas fell away; look at John 6:64.) "After this, many of his disciples drew back and no longer went about with him" (John 6:66).
This is the only record we have of any of Christ’s followers forsaking him for purely doctrinal reasons. If it had all been a misunderstanding, if they erred in taking a metaphor in a literal sense, why didn’t he call them back and straighten things out? Both the Jews, who were suspicious of him, and his disciples, who had accepted everything up to this point, would have remained with him had he said he was speaking only symbolically.—Catholic Answers
Might it be said that any community that refers to itself as Christian has the burden of proving that it belongs in or to the fold of the one Church that Jesus founded? A sure proof of the validity (or invalidity) of one's community as one that belongs to Jesus is whether or not it affirms that the Holy Eucharist is the Real Presence of Jesus Christ. Might it be said, then, that lacking that appreciation or teaching, a community cannot be said to be in the fold of Jesus' Church. Only a community that possesses valid Holy Orders can be said to be a church, and only such a church can have a valid Eucharist. Can an ecclesial community be Christian which rejects the Real Presence?
And (Jesus) did not do many mighty works there, because of their unbelief.—St. Matthew 13:58A priest is changed at ordination. That ontological change which occurs in the man to be ordained is effected by the Holy Spirit. The hands of the ordained priest become the hands of the Resurrected Jesus through which Jesus transforms the bread and wine into His Body and Blood. "Do this" is the moment during the Last Supper that Jesus established the Sacrament of Holy Orders (cf. St Luke 22:19; 1 Cor. 11:24). It takes an validly ordained priest to "do this" as Christ intended His priests to so do. Only a validly ordained priest can become the vessel through whom Jesus will change the bread and wine into His Body and Blood. Though God is not bound to His sacraments and can act in extraordinary ways to reach sinners, it is unlikely that He would contradict Himself by enabling heretics to parody His gift of the Eucharist, heretics who reject His teaching and think that the Lord was merely talking about symbols and metaphors at the Last Supper.
The reason why Catholics celebrate an unleavened Eucharist is very simple. We just never changed it from the day of the Last Supper. We have no problem with the Eastern tradition of leavened Eucharist, and its validity, and we have no desire for the east to change. But we also have no desire to change our unleavened host.—CB
Dear priests—have faith! Through you, Jesus gives us the greatest miracle. Trust that God will bless and heal his people through you! If Jesus can transform wine into His Blood and bread into His flesh, Jesus can cure every manner of disease, heal every affliction of the body, mind and spirit through those who, docile to His Person and teaching, believe in the Real Presence! Believe, therefore, in the grace of God that Jesus gives to those who, trusting in His promise to be with them, trust in His transforming grace. Without Jesus, we can do nothing. With Jesus, we can move mountains (cf. St. Matthew 17:20) and heal the infirmed. May Jesus find willing vessels through whom He may heal the sick.