The religion of Zoroaster, in the context of Eran Shahr (Land of Iran/Arya), has 3 distinct phases.

There is the pure Zoroastrianism of the early Hakhamanish Empire and in particular Dariush the Great who was single-mindedly devoted to Ahura-Mazda [1], the name by which the prophet Zoroaster knew God. (It means mighty-wise). His predecessor, the more widely known Cyrus the Great however appears to have been rather relaxed regarding the deity and gave lip service to both Marduk and "god of Israel", depending on which of the subject nations of his empire he wanted to please. Both, however, seem to have had no need for the intermediary of a priesthood. The fire temples of that era (Azar-Padgah, today Azer-Baijaan, and as it with everything apparently "Turkish" /g) are a testament that some form of Zoroastrianism was in fact practiced. The distinction here is that one does not hear much about priesthoods. This empire's kings however very rapidly, it seems, descended into polytheism with various Fereshtah (Angels) showing up alongside of Ahura-Mazda.

There then follows the rather lengthy period of Hellenic Persia (the Selucids) and the subsequent Iranic (but not Persian) empire of the Parthians which over time shed the imposed Hellenic cultural elements. It is not clear (to me at least) to what extent Zoroastrianism was prevalent in this era or how it was practiced.

The second great Persian empire (& the 3rd Iranian) one was the Sassanids. They very much sought to project themselves as the true inheritors of the Hakhamanish King of Kings, and Zoroastrianism was a very pronounced feature of this era in Iran's history. Regrettably, this is also when institutionalized priesthood [2] became firmly established in Iranian society. A particularly nasty specimen was Kartir [3], a very powerful and influential 'high priest'. This was not a tolerant era in Iran. Fundamentalist, mired in ritual, and brutally suppressive of any competing religion. Christians, Mazdakians, and Manichians were violently suppressed during this era. It is important to note that during this empire the original Zoroastrianism seems to have been a trace memory - many of the secondary scriptures attributed to Zoroaster - the name means Golden Star or the Sun btw - were in fact written in this era.

By the time of the advent of Islam, it appears the religion had decayed sufficiently, to wit, the famous companion of prophet Muhammad, Salman the Persian, was actually the son of a high ranking Zoroastrian who first sought spiritual meaning in Christianity and then directed by the Christian monk that he followed to go seek the prophet that was highly anticipated in that era, and this of course was Muhammad.

The Parsis are the descendents of the Zoroastrian (elite) that fled the Arabs into what is now known as India. Many centuries later these same diaspora Persians, now Parsis, sent instruction back to the remnant in Iran to re-educate them in the religion. In this era, naturally as a minority religion in both Iran and India, Zoroastrians were obviously not intolerant.

As to the ridiculous notion [4] that Zoroastrians at any period could remotely be considered "devil worshippers", let us note that the Abrahamic religions owe the Angels, Heaven, Hell, Judgement, and "a New Earth" (for the "meek") all to the Iranian prophet and his ministry. Even the Arabic Qur'an uses Persian theological terms such as "deen" (religion) and "ferdows" (Pardis now Paradise) and in it Allah insists that "He is al-Aziz al-Hakim" which is the precise formulation of Ahura-Mazda in Arabic ...

[1]: https://en.wikipedia.org/wiki/Behistun_Inscription

https://en.wikisource.org/wiki/The_Sculptures_and_Inscriptio... [grep for ahuramazda - 76 instances. Dariush was devoted, without doubt.]

[2]: https://en.wikipedia.org/wiki/Mobad

[3]:https://en.wikipedia.org/wiki/Kartir

[4]: https://en.wikipedia.org/wiki/Names_of_God_in_Zoroastrianism