Convert, slowly move to monotheism.
(Nature spirits --> Earth mother ---> greek/roman-style pantheon ----> monotheistic god in whose name we shall commit horrible atrocities! )
That's not really how it worked though, the Hebrew were fairly monotheistic from the beginning, it was the Diaspora that started converting everyone else.
Then christianity took over. It was almost usurped by Islam, but Islam began too late, I think. The enlightenment hit before it could convert enough people to be bigger than Christianity.