Certainly the entire book of Genesis is allegorical
then why does the rest of scripture treat Genesis as the historical record? The simple fact is that scripture doesn't treat the account Genesis any differently than it does any other historical book of the bible. In fact, I could easily argue that Genesis is treaty as the blueprint for the rest of the bible.
Can the events in Genesis be taken allegorically? Absolutely. For example, a lot of the events of Joseph's life establish a pattern that the Messiah would later follow. But that doesn't make the events of Joseph's life any less real.
Scripture simply treats the events recorded in prior scripture as BOTH literal and figurative.---
, although certain parts (Joseph going into Egypt) can be taken as actually happening.
Well, how could Joseph be real but not his father (Jacob), his grandfather (Issac), or his great-grandfather (Abraham)?
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In all kindness, I don't think you have thought this through.