You don't need to read too much of the New Testament to know that Jesus was quite adept at using strong, harsh language to describe spiritual truths. He tells us to cut off our limbs and gouge out our eyes if we sin (Matthew 18:8-9; Mark 9:43-48). He tells us to hate our families, or we cannot be His disciples (Luke 14:26). One of the strangest statements He makes is "Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you. Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day." (John 6:53-54).
If you're a Catholic, you might argue that this is the one time Jesus literally meant we must eat His flesh and drink His blood. After all, your religion does teach that when the priest consecrates the bread and wine consumed during communion, it literally becomes the flesh and blood of Jesus, and you consume it as a "sacrifice of thanksgiving to the Father" (Catechism of the Catholic Church 1360). This, you call Transubstantiation.
One argument Catholics use to beef up this interpretation is the disciples' response to the statement. "This is an hard saying; who can hear it?", they said (v60). And of course, verse 66 tells us many of them left Jesus from that point on. Game set and match, "Protestants". Case closed, we lost. Let's all go submit to the Pope. Unless...
When we expand our reading of John 6 from a few mismatched verses to instead cover verses 25-71, we see that this interpretation is so embarrassingly flawed, it makes us ask if Catholics have ever actually read the chapter they rely so heavily on to prove their point. To begin with, in verse 63, Jesus tells us "It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life."
Jesus, with the view of ordering the state of salvation as a spiritual thing, set out with the principle, It is the spirit that quickens; and then added, The flesh profits nothing — meaning, of course, to the giving of life. He also goes on to explain what He would have us to understand by spirit: The words that I speak unto you, they are spirit, and they are life. In a like sense He had previously said: He that hears my words, and believes in Him that sent me, has everlasting life, and shall not come into condemnation, but shall pass from death unto life. Constituting, therefore, His word as the life-giving principle, because that word is spirit and life, He likewise called His flesh by the same appellation; because, too, the Word had become flesh, We ought therefore to desire Him in order that we may have life, and to devour Him with the ear, and to ruminate on Him with the understanding, and to digest Him by faith.
The above paragraph is not my own. I changed the word "He" to "Jesus", but the rest of those very "Protestant"-sounding words come from another source. A source older than me, a source older than Martin Luther, a source older even than the Catholic Church. These words were written by a Church "Father" named Tertullian before 220 A.D., showing that a more symbolic interpretation of "eat my flesh" existed long before the Reformation, and even before the Catholic Church claims to have given us the Bible. This blocks one of Catholicism's most cherished arguments: The argument from 1500 years of unanimous Christian tradition prior to the Reformation. It is clear from Tertullian alone (though many other historical figures prior to the Reformation disagreed with the literal interpretation) that the symbolic interpretation is neither new, nor entirely unfeasible.
As it turns out, however, there is more to John 6 that favors Tertullian's interpretation, starting with the fact Jesus actually explained what He really means. Pay special attention to verses 35-40: "I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst. But I said unto you, That ye also have seen me, and believe not. All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out. For I came down from heaven, not to do mine own will, but the will of him that sent me. And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day. And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day."
This continues (or, more accurately, begins) the analogy of Jesus being the bread of life. But notice He says he who comes to Jesus will never hunger, and he who believes on Him shall never thirst. Those who do so, Jesus says He will by no means cast out, and that everyone who sees and believes in the Son will have everlasting life, and Jesus will raise him up on the last day. At this point, physical consumption of food (which would be a fleshy thing, yet verse 63 reminds us the flesh profits nothing) is not even in view here. The last supper has not happened, nor is the ritual even being placed in the disciples' minds.
In fact, one critical difference between the two events is the direction of the analogy. In John 6, nothing but faith is being discussed. This makes sense. Throughout all of scripture, faith is both the requirement and guarantee of salvation. There are many strange ways in which Jesus represents Himself in this way. He calls Himself the true vine in whom we must abide (John 15:1-4), or the door (John 10:7), neither of which are literal, nor do the Catholic Church claim they are. It should not surprise us that Jesus, who so often spoke in figurative language, would figuratively describe Himself as the bread of life, whom we must eat for eternal life.
But when it comes to the Lord's Supper, neither He, nor any New Testament author, suggests this is anything other than a symbol. Do this in remembrance of me (Luke 22:19, cf. 1 Corinthians 11:24-25) is quite explicit. It is a ritual of remembrance, and is nowhere linked to eternal life. When people ask "what must I do to be saved?", the response is never "consume the literal flesh and blood of Jesus". Rather, it is believe in the Lord Jesus (Acts 16:30-31).
The Catholic interpretation of John 6 is demonstrably wrong. Although there are some Church "Fathers" who seem to have erroneously held to some primitive view of Transubstantiation, and still others can be twisted to sound like they also did, the fact is the Bible disagrees, even to the extent of recording Jesus' own words to do it. "I am the bread of life" must be taken as figuratively as "I am the true vine", and "I am the gate of the sheep". That is, it is strange (and therefore memorable) language designed to convey a spiritual truth. Jesus' words are spirit, not always literal, the proof of which can be seen every time a priest consecrates the bread and wine, for bread and wine it remains. No Catholic, no matter how sincere his faith may be, has ever eaten the flesh of Christ, nor drank His blood, for He currently sits at the right hand of the Father until He makes His enemies His footstool (Hebrews 10:12-13), and commands us, in no uncertain terms, to believe no one who says He has returned to the Earth, even if they show some sign or wonder (Matthew 24:23-27). Therefore, let us do what He did command us to do: Eat His flesh by coming to Him, and drink His blood by believing in Him.