"Nineveh never converted; there's just no evidence of that."
Jonah 3:10 above states that "they turned from their evil way". For how long? I don't know. It was atleast long enough to spare them from:
what he's saying is that there is no supporting evidence that nineveh ever adopted judaism.
unfortunately, the bible is just not very trusted as a history even at it's more historical (kings+chronicles). and jonah is just not history. it's a fable, with a clear moral to demonstrate. it's literature, not documentary.
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