Cynical bear's observation was 100% spot on. Literal one verse, allegorical/spiritual the next, all depending on which doctrine Catholics are trying to support. It's inconsistent, and disingenuous, and intellectually dishonest.
The whole passage or discourse should be interpreted in the same manner. If it can't be then it needs to be interpreted differently. Since the discourse in John 6 makes no sense, cannot be interpreted literally for the whole thing, then it must be interpreted differently, which does make sense and make it internally consistent with the rest of Scripture.
So when Jesus says you have to eat His flesh and blood to have life in you and that when you eat you will never die or never hunger or thirst again, it's more than hypocritical to take the eating flesh and blood literally but not the living forever and never being hungry or thirsty.
Actually, only a carnal people would think such and that is what Augustine says.
One must first believe and then know.
So, those who believed Jesus is the Son of God, then knew that what He said is true, whether they could understand it or not.
If one does not understand the union of the carnal and the spiritual, it is because one does not believe that Jesus could give us His flesh to eat and His blood to drink.