The manna in the desert fed over one million Jews every day for forty years. From the biblical narrative, we can estimate that roughly 300 rail train cars full of manna came to the camp each day (twice as much on Friday and non on Saturday). And yet, Jesus says that the bread his Father gives through him makes the miracle of the manna to be nothing at all in comparison.
Could a mere symbol be a greater miracle than the manna? No. The Eucharist is greater, because the bread has changed into a man who is God. And, as God infinitely surpasses the whole universe, so too does the Eucharist infinitely surpass the manna.
Four reasons why the Bread of Life Discourse cannot be a metaphor:
1) Jesus doesn't use metaphorical language (he says, "Truly, Truly I tell you", "indeed", "true food", "true blood", etc.).
2) The people listening thought Jesus meant it literally, and our Lord doesn't correct them.
3) The metaphor "eating flesh" is used in Scripture to mean "to attack", "to persecute", "to hate", etc. Thus, it would be a bad thing to "eat Jesus' flesh" if we were to mean it metaphorically.
4) The earliest Christians never thought it was a metaphor but understood the Bread of Life Discourse in relation to the Last Supper.
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