“A lot of Americans are otherwise bible-believing Christians who just understand the opening chapters of Genesis to be metaphorical.”
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Those “Christians” have painted themselves into quite the corner, since Jesus did not believe the “opening chapters to be metaphorical”. In fact, if they were metaphorical, there would be no need for Jesus. As I said, such “Christians” have really got themselves one doozy of a catch-22.
Quite right: either all of scripture is true, or none of it...
4 And Jesus answered him, saying, It is written, That man shall not live by bread alone, but by ~every~ word of God.
16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
So, when God says there was evening and there was morning, the first day...sad that people really think God does not understand the ramifications of what he said or does not mean what he says.