Zoroastrianism is a principle component of Iranian identity in the antiquities, especially under Achaemenid rule during which its doctrines were formalised and the church was tied to the state apparatus.

As the empire grew, Zoroastrianism was spread as far as India (I heard the biggest Zoroastrian community today is located there). Some vassal states within the Acahemenid/Sassanid spheres of influence even adopted the religion, such as Armenia before it converted to Christianity. The extent of Zoroastrianism was so deep-rooted in Iranian societies that even centuries after the Islamic conquests it remained prevalent (There wasn't forced conversion to Islam but many eventually did to avoid paying war tributes).

There was also a popular movement led by the Zoroastrian priest Mazdak who preached for an egalitarian social system and the distribution of the wealth of the nobility and priests to peasants.