I suppose you would also argue that the body of Peter was transubstantiated into Satan in this verse? Either Peter is literally Satan, or Jesus is lying... right?
The reason this line of thinking does not work against Catholicism is because we know that people are capable of speaking symbolically sometimes and literally in other times. (Although in your example, I believe there are exegetes who believe Jesus was literally addressing Satan influencing Peter's speech, but I digress since there are other examples that you could have used such as Jesus saying "I am the door" (Jn 10:9). It's hardly sensible to say that if Catholics believe Jesus to have been speaking literally in John 6 that they therefore cannot think Jesus is capable of speaking symbolically somewhere else. That makes no sense to impose that rule on the Catholic.
There are a variety of reasons why John 6 should be understood literally. Let me touch on one or two. In Catholic theology, we relate the New Testament in light of the Old Testament. This is called typology. You see Paul speak of this in Romans 5 when he identifies Jesus as the superior antetype of Adam. You see the author of Hebrews (ch 8) speak of this when he compared the sacrifices of the OT to the corresponding superior sacrifice of Christ. You also see Jesus speak of this earlier in the John 6 discourse when he spoke of the bread that fell from heaven. One thing you will always see in the order of typology is that the New Testament antitypes are superior to their Old Testament types. Jesus is superior to Adam. Christ's sacrifice is superior to the OT sacrifices. And the bread of life in the NT is superior to the bread that fell from heaven in the form of manna.
Now, if we apply this "it's just a symbol" rationale to John 6, we cause a fatal problem in the order of theology. The NT "bread" suddenly becomes inferior to the OT manna. After all, the OT manna was 1) of supernatural origin and 2) of benefit for temporal life. When we insist the bread in John 6 is "just a symbol" we make it worse than the OT manna because we say its origin is less-than-supernatural and we deny that it is of benefit for
eternal life. In other words, a symbol-only interpretation of the John 6 bread renders it the inferior type to the OT manna, which Christ made the typological comparison to in verses 49-51.
Secondly, the audience themselves understood Jesus literally. They walked away thinking he wanted them to eat his flesh. If, as the "symbol-only" folks argue, Jesus was speaking symbolically as he often did, then of course the reaction of the audience would have been: "Oh! He is speaking symbolically as he often does!" It makes zero sense for the audience to suddenly take him literally if he were speaking symbolically as he often did, and as his followers heard him speak before.
That's just two reasons why we can understand the bread of life as literally Christ himself, and that's just if we look at John 6 alone----one of a multitude of Scriptures regarding Christ and the Eucharist.
Rick, please don't think I'm picking on you, but if this is to be taken figuratively, then should not everything Christ said be taken that way? ...And if that's the case, then how are we to believe that He said anything litterally, ever?
