Zoarastrianism is not monotheistic. Classic Zoarastrian belief was dualistic. That is, it believes in two equal but opposed gods, one good, one evil.
More modern Zoarastrianism is, as I understand it, a kind of panentheism wherein the good god is seen as being an integrated part of the universe that he is continually creating. I am no expert on modern Zoarastrianism but I think I recall that it borrows heavily from a particular sect of Islam that was, at one time, itself heavily influenced by classical Zoarastrianism.
The “duality” found it’s way into Jewish thought after the Exile. “Satan” became more of a figure like Ahrimahn, Mazda’s “shadow figure”…while the “duality” is thought to be “equal” in many ways…good will one day win out, and evil will be vanquished.
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