What the Bible says about Pharaoh's Opportunity to Repent
(From Forerunner Commentary)
God's warning to the Egyptians that He would kill their firstborn was part of the plan from the beginning. It was not, as The Ten Commandments movie would have one believe, a last minute decision to which God resorted when all the other plagues failed to achieve His desired effect. In His mercy, God repeats His warning to Pharaoh, giving him plenty of opportunity to repent: "[A]ll the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh who sits on his throne, even to the firstborn of the maidservant who is behind the handmill, and all the firstborn of the beasts" (Exodus 11:5).
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The Law of the Firstborn