If you could erase one religion from ever existing, which would it be? That means nobody will ever be that religion or know about that religion, and all religious actions in the past by members of that religion will go away (Example: If you vote Christianity, the Crusades never would have happened. If you vote Islam, 9/11 never would have happened).
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I would wipe out half of them, and divide the world into those who are part of a religion, and those who are not. Then I would document how each group developed over the next fifty years.