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In the Lux Invicta mod for Crusader Kings II, there is a massive myriad of different religions, ancient, medieval or hypothetical, to worship in-game. One of these that struck out to me (and forms the basis of this thread) is one Mithraic-offshoot religion known as Eosphorism. I won't get too deep into this hypothetical religion, but it basically worships Lucifer as the bastion of light against Christians and Jews alike. (Some info can be found here.)
Now what could make this hypothetical religion a reality and widespread power in history?