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Malachi wrote his letter c. 400 BC as a warning Israelites had fallen back into their old ways of worshiping other gods.
Ask the Mormon how Nephi wrote basically the same thing around 588 BC. The intro for 1st Nephi chapter twenty-two gives the dates of when this supposedly happened. Also, what would this warning mean for the Nephites in 588 BC?
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1 Nephi 22:15 “For behold, saith the prophet, the time cometh speedily that Satan shall have no more power over the hearts of the children of men; for the day soon cometh that all the proud and they who do wickedly shall be as stubble; and the day cometh that they must be burned.”