Conversion of the Iranian population from Zoroastrianism to Islam did not happen quickly after the conquest, and historians estimate that a Muslim majority did not emerge in most of Iran until several centuries later, with substantial Zoroastrian communities persisting, particularly around Yazd and Kerman in central Iran, well into the medieval period and beyond. Economic and social pressure, including differential taxation and periodic episodes of more active persecution under particular rulers, worked over generations to reduce the Zoroastrian population relative to the growing Muslim majority, rather than eliminating the community outright at any single point.
Some of the pressure driving conversion was practical rather than doctrinal: the jizya tax fell only on non-Muslims, and various administrative and social advantages accrued to Muslims under Islamic rule, incentives that operated cumulatively over long periods to draw individuals and families toward conversion even without direct coercion in most times and places.
The community that remained committed to Zoroastrianism inside Iran continued to maintain its religious institutions in reduced form for the rest of the medieval and early modern period, while a separate branch of the community had already begun, by tradition several centuries after the conquest, the migration to India that would produce the Parsi diaspora, ensuring that organized Zoroastrian religious life continued outside Iran even as the community within Iran grew smaller relative to the surrounding Muslim population.