“the only sources for the early (pre-Achaemenid) history of the eastern Iranian peoples are the Avesta, the Old Persian inscriptions, and Herodotus. … - Prods Oktor Skjaervo

The pantheon was never eliminated, and Zoroastrianism, in some sense at least, remained a polytheistic religion throughout its history, although today the many deities have lost their individual divine character and are not worshipped for themselves but have been reinterpreted as allegories or symbols. Thus, modern Zoroastrianism is probably best described as monotheistic, certainly as monotheistic as Christianity with its Trinity and angels, though less monotheistic than Judaism and Islam.

Since the classical Greeks already, it has been common to date Zarathuštra to the 6th century BC, hardly a few generations before the Persian wars. In popular literature, this date is still given, but scholars have now settled for an earlier date: “The archaism of the Gāthās would incline us to situate Zarathuštra in the very beginning of the first millennium BCE, if not even earlier.” (Varenne 2006) But how much earlier? According to leading scholar SkjaervØ, “Zoroastrianism (…) originated some four millennia ago”.

“The earliest mention of Iranians in historical sources is, paradoxically, of those settled on the Iranian plateau, not those still in Central Asia, their ancestral homeland. ‘Persians’ are first mentioned in the 9th century BC Assyrian annals: on one campaign, in 835 BC,… There are no literary sources for Iranians in Central Asia before the Old Persian inscriptions (Darius’s Bisotun inscription, 521-519 BC, ed. Schmitt) and Herodotus’ Histories (ca. 470 BC). ...