In its origin Zoroastrianism is not monotheistic. It is a widespread myth that Zoroastrianism is said to have shaped Judaism in monotheism. Zoroastrianism was not a static system of thought but developed over many hundreds of years. It originated as an anti-daēuua movement in the late second millennium BCE promoting the Yazatas over the traditional gods; the ancient Persians inherited this worldview by using the neologism bag as a term for gods worthy of worship. Eventually the Yazatas became viewed as forms of the creator Ahura Mazda; this process of pantheon reduction was replicated elsewhere in the ANE. For example, in Assyria the entire pantheon was thought by some to be varied manifestations of the national god Aššur, as implied in the name Gabbu-ilāni-Aššur which means “All Gods Are Aššur”. Almut Hintze discusses the kind of monotheism in the Young Avesta (c. 900-600 BCE, with the latest portions before 300 BCE) in this article.

In summary, it was considered “perfectly legitimate to worship any of Ahura Mazda’s spiritual and material creations because ultimately they derive from him and comprise his substance. One worships Ahura Mazda by worshipping his creations”. Whether monotheism is an appropriate label for this theology depends on how one defines the term. But it is worth noting that Deutero-Isaiah, who was responsible for some of the earliest unequivocal statements of monotheism in Judaism, may have drawn on the pre-YAv Gathas (see the article by J. Blenkinsopp in CBQ, 2011), and thus may have been influenced by Persian ideas (as was his use of Teispid propaganda). But his form of monotheism was very different from the concept of divinity in YAv liturgy, and there is nothing that cannot be explained by inner-Jewish developments.