Some of the typical arguments go like this:
At the Last Supper, when Jesus broke the bread and drank the wine, He was alive. His instruction was to do this (break bread and drink wine) in "remembrance" of him when they come together. He made no comment that would imply a need to change the substance either then or on the altar. The word consecration doesn't mean a change of substance, it's a solemn commitment, dedicating something to God. He was asking them to gather together sharing the bread and wine, like they were doing then, remembering their time with him, remaining dedicated to God. Bread and wine were symbols like other symbols such as: Land flowing with milk and honey, Kingdom of priests, the Cedars of Lebanon, Lamp, Footstool, Shadow of God's wing, Hearts of stone, Stand in the gap, Shake dust off feet, Whitewashed tombs, Capstone, Slave/servant of Christ, Heap burning coals on an enemy's head, Serpent, Dove, Donkey, Lion, Wolves, Dog, Sheep and 6 other animals. Metaphors, not meant to be taken literally. The Apostles never "consecrated" the breaking of bread, and transubstantiation was as foreign to them as it is too much of the world today. All the conventions of Hebrew thought, of the parallel which we find in the Old Testament, would have prevented such a literal understanding. The words could only have been understood symbolically.
Jesus used a lot of different teaching styles in his ministry. Jesus often spoke symbolically. He frequently used hyperbole, exaggerating to make a point. And He also spoke literally. So how do we know when he spoke literally as opposed to symbolically? There are many clues in the context as well as the presentation. For example, when Jesus speaks in parables, frequently the Apostles come to him, not understanding, and Jesus explains the meaning to them. On the other hand, when speaking literally, it is very common for Jesus to repeat himself, stressing the veracity of his words.
Knowing this, we can take a look at the teaching of Jesus surrounding the institution of the Eucharist, or Holy Communion. In John 6 in the Bread of Life discourse, Jesus prepares us for what's coming at the Last Supper. But at this point the Apostles and the other disciples do not know what's coming.
In John 6, Jesus tells us that we must eat his flesh and drink his blood in order to have everlasting life. He repeated himself, not just one more time, but six more times. Never in any passage has Jesus reiterated something seven times. These words were very distressing to His followers. In fact these words were so difficult that in John 6:66, what I often refer to as the saddest verse in the Bible, many of Jesus' disciples abandon him. The disciples find this teaching too hard to accept. Even today, many Christians refuse to accept these words, believing that they are only symbolic.
If Jesus were speaking symbolically, he could have easily clarified and explained what he meant. But instead, he stands firm, he turns to the Apostles and asks them if they want to leave too.
Jesus wasn't just speaking symbolically, he was being very clear. He repeated himself over and over again. You'll find that it's frequently the case in the Gospels that when speaking figuratively, Jesus later explains himself to the Apostles. But when speaking literally, Jesus repeats himself. Here Jesus states his position, not just once, not just twice, but seven times.
Let's pick up at John 6:48.
Let's focus for a moment on verses 66-68. Even after being abandoned by many followers in verse 66, Jesus didn't try to stop them and bring them back. He didn't explain himself, or that he was speaking symbolically. Rather he stood firm, drawing a line, challenging them directly.
Why did the disciples abandon Jesus over these words?
There are a couple of reasons worth looking at.
First, there was a symbolic meaning in the words. In the time and culture of the Jewish people present, to eat someone's flesh and drink their blood were words that would indicate a form of persecution. It would have been like "taking a pound of someone's flesh" only worse. It would have been a horrible thing. Imagine Jesus saying, In order to have eternal life, you must persecute me. It doesn't make sense, does it? Jesus was a very good teacher. He spoke in many different ways during his ministry. If Jesus were being symbolic, he could have found better language, don't you think?
Second, the language Jesus used was very descriptive. It doesn't translate well into English, but in Greek there were some very definite words indicating when he said to eat his flesh, he meant eat his flesh. He spoke using words such as "chew" or "gnaw." If that's not literal language, I don't know what is. Jesus gave us His Body and Blood, and he used literal language when he did. But it was harsh. In doing so, He scared off many of his disciples.
It wasn't until the Last Supper that the words of Jesus were made clear to them.
The words of the Eucharist appear in the Bible four times.
In each case, never is the word symbol used. In fact, in the very next verses in St. Paul's letter, 1 Cor 11: 27-29, St. Paul confirms the seriousness of Jesus' words when he tells us:
Consider also, one chapter before, in 1 Cor 10:16-21:
Here St. Paul also refers to the bread being a participation in the Body of Christ. But notice also that St. Paul makes a comparison to the Altar. The Altar is a place of sacrifice. By using this comparison here, St. Paul is indicating that this bread is a sacrifice. Regular bread, served at a meal, wouldn't be a sacrifice on an altar.
We've established that this is a hard teaching. Why does Jesus present himself in this way? We know that all of Jesus' teaching is part of laying out His plan. Jesus is establishing the New Covenant. But the New Covenant is a fulfilment of the Old Covenant.
If we look at the Bible as a whole, we can follow a progression. The Bible as a whole is the story of God's relationship with His people. He establishes a covenant very early on. And as time progresses, the plan that God has, that makes up the covenant, grows. God starts small as "the people" are young. And as the people grow and can accept more, God reveals more and more of his plan to His people.
Recall, when Abraham had a son, his beloved son Isaac, Abraham believed that he was supposed to sacrifice his son for God. He took his son to a hilltop to be sacrificed. The son carried his own wood on his back to the place of sacrifice. Then God provided an alternate sacrifice, a ram. Does this sound familiar so far?
As the Old Covenant plan was revealed, God instituted the Passover meal, with unblemished lamb or goat to be sacrificed and eaten with unleavened bread. And God provided manna, bread in the desert to sustain his people. There are parallels from Abraham to the Passover.
In the New Covenant the parallels continue. God's plan called for the sacrifice of his only begotten son, (like Abraham's son Isaac) who was taken to a hilltop, (like Isaac) carrying his own wood (like Isaac). But Jesus is the Lamb of God, (like the ram that was sacrificed in place of Abraham's son, Isaac.)
Further, Jesus, the unblemished Lamb, becomes our Passover meal. The Lamb's blood marks the door frame, as Jesus' blood marks the Cross. Jesus becomes the Manna, the Bread that not only sustains us, but gives us eternal life.
In Biblical typology, nothing in the Old Covenant can be greater than its fulfilment in the New Covenant. The sacrifice, the Passover meal, and the miraculous bread, as given to us in Jesus, must be greater in the New Covenant version than in the old.
If God can give us these things, manna in the desert, and feeding 5,000 with five loaves and two fishes, then certainly Jesus can give us his Glorified Body in the Eucharist.
Yes, this is a hard teaching. It's so hard in fact that Jesus was willing to let some of his followers leave him without backing down. Jesus speaks literally because he is literally giving us the New Lamb, the New Manna, His own flesh and blood.
This hard teaching is a teaching that has been believed by the Church throughout its history. The Apostles believed it, the writers of the Gospels and St. Paul believed it, the early Church Fathers believed it. The early Christians believed it. Until the reformation took place 1,500 years later, it was a core belief of all Christians.
It is still believed today, by Christians who understand and are taught the fullness of the Church. The true Eucharist is still celebrated every day, at every Mass, in Catholic Churches around the world.