I've studied the Achaemenid Empire and have read in some source a while back that Armenia converted to Zoroastrianism under the Achaemenid regime. This got my mind going as Armenia also converted to Christianity when that faith became popular. What's notable about these religions is that they're both Axial Age religions. While Axial Age religions often converted pagan or pre-Axial religions into their faith as a general rule Axial Age religions rarely ever overtake other Axial Age faiths. After this I started thinking about the Arab conquests of 7th century and how the majority of Iran converted to Islam by the 9th or 10th century despite having a millennia old religion that had dominated the region since the inception of Iran.
These ideas coming together make me ask for the nature of Zoroastrianism. I as an uneducated history nerd came to the conclusion that either this religion was more of a religion of the elite/nobility in the religion's frontier, barely followed by the masses. And/or there is some trait about Zoroastrianism that makes it easy for its followers to convert to other religions despite its monotheism.
I'd be very interested to see what professional historians have to say!
Ok, there are a couple of misconceptions in here. The Axial Age is often overstated, and doesn't truly apply to Christianity or Zoroastrianism. Additionally a great deal of what defines the Axial Age according Karl Jaspers (who coined the term) includes pagan Greek philosophers like Socrates and Aristotle. It is not a term meant to designate religion so much as philosophy.
Your point about Axial Age belief systems rarely being overtaken is easily disproved by Christianity. The Axial Age conventionally refers to the 8th-3rd centuries BCE to cover the rise of Socratic philosophy, Confucianism, Daoism, Judaism, Buddhism, etc. in the same general time period. Christianity adopted aspects of Socratic/Platonic philosophy, but utterly overwhelmed them and converted huge numbers of Jews after its foundation in the 1st century CE, 300 years after the end of the so-called Axial Age. Likewise, different regions of India have fluctuated between Buddhist and Hindu majority throughout their history.
Zoroastrianism's inclusion in the "Axial Age" is a product of some early debate about the lifetime of Zarathustra/Zoroaster that was still in play when Jaspers was developing the idea in the early 20th century. Both late Roman and Sassanid Persian sources placed Zarathustra some time in the 7th-6th century BCE, just before the rise of Cyrus the Great and the Achaemenid Empire. Modern linguistic analysis of the Gathas (the section of the Avesta attributed to Zarathustra) places his lifetime in the range of 1200-1000 BCE, centuries before the date used by Jaspers and his contemporaries. Both theories were in circulation 100 years ago, but the c.1200 date is more widely recognized today. However, Avestan religious hymns and prayers continued to develop into the 5th-4th centuries BCE.
All of this said, Zoroastrianism was quite widespread in Greater Iran by the 2nd-3rd centuries CE. Buddhism had some dominance in the east, and Mesopotamian polytheism continued to hold sway in Mesopotamia, but in Iran and Central Asia, Zoroastrianism was largely unchallenged at its height.
In the Achaemenid period, things were quite different. First of all, there's lots of scholarly debate about whether or not we can even call their religion Zoroastrian. I'm personally of the opinion that we can, but others are hesitant because we lack any information about specific dogma (in which case they tend to call it Mazdaism). Zoroastrianism was spreading west, but it's unclear how much or if it ever reached below the ruling class during the Achaemenid period. The Achaemenids did not generally seek to impose their beliefs on subject peoples (a practice that was generally pretty rare up to that point anyway). However, there is some evidence that they did enforce worship of Ahura Mazda, and presumably other related beliefs, on Iranian peoples, including the native Elamite population of Parsa and Susa.
In the Behistun Inscription, Darius condemns the Elamites and Scythians for not worshipping Ahura Mazda, which he does not accuse the other rebel peoples of. Likewise, Xerxes condemns "Daivadana" (Daiva Temples) in inscription XPh, evidently describing a rebellion by people who worshipped deities the Persians considered "daiva," the false gods or demons of Zoroastrian tradition. However, extremely un-Zoroastrian practices were also tolerated in the same region, like the worship of traditional Elamite gods in the royal cities of Persepolis and Susa.
After Alexander the Great's conquests, we really don't have a clear picture of Zoroastrian development. It has even been debated whether or not the Arsacid kings of Parthia were Zoroastrian. We know from Strabo that there was a signifcant Zoroastrian population in Capadoccia by the first century, and Zoroastrian influence in Armenia is well documented with both fire temples and references to Zoroastrian divinities like Ahura Mazda, Mithra, Anahita, or Aramaiti.
Aremenia itself is an interesting example because Zoroastrianism seems to have coexisted alongside a local polytheism that heavily borrowed from Zoroastrian tradition. Ahura Mazda (Aramazda), Anahita (Anahit), Mithra (Mihr), and many other divinities were incorporated into the Armenian mythological pantheon, with Aramazda taking on the rule of king of the gods, including gods derived from native Armenian and ancient Urartian traditions.
Notably, despite usually falling outside the control of the Arsacids and Sassanids, both Armenia and Capadoccia had maintained Iranian rulers through the Hellenistic Period, giving Iranian influence more time to take hold in those regions.
So that's the picture by the mid 3rd century CE. Greater Iran is almost entirely Zoroastrian, with other local traditions continuing on the edges of early Sassanid control, but the seeds of decline were already there. Buddhism was making inroads beyond modern Afghanistan, Cappadocia was already an important center of early Christianity, and of course Armenian conversion was just around the corner in 301.
The 4th century and beyond would see a slow decline for Zoroastrianism as Buddhism spread rapidly along trade routes out of India and into Central Asia while Christianity spread rapidly in the west as it became more accepted in the Roman Empire. Meanwhile conversion came from inside Zoroastrianism itself with Manichaeism and its prophet, Mani, briefly receiving royal patronage in Iran. Then again in the 6th century with the less popular Mazdakism, or sort of proto-communist revision of Zoroastrianism.
