Zoroastrianism had held the position of state supported religion under the Sasanian dynasty for more than four centuries, with the crown, the priesthood and the wider religious establishment closely intertwined in the administration of pre-Islamic Iran. This arrangement ended with the Arab Muslim conquest of the Sasanian empire, a process substantially complete by 651 CE with the death of the last Sasanian emperor, Yazdegerd III, though resistance and instability continued in various regions for some time afterward.
Under the new Muslim administration, Zoroastrians were generally classified among the protected religious communities required to pay a special tax, the jizya, in exchange for a degree of legal toleration to continue practicing their faith, a status that fell short of full civic equality with Muslims and that varied considerably in its practical severity across different periods and regions of Islamic rule. Zoroastrian fire temples, priesthood and religious institutions continued to operate in many parts of Iran for generations after the conquest, but without the direct state sponsorship and legal primacy they had enjoyed under Sasanian rule.
The loss of state patronage is generally treated by historians as the decisive structural turning point after which Zoroastrianism's long term decline as the majority religion of Iran became effectively irreversible, even though the process of decline itself unfolded gradually over many subsequent centuries rather than immediately.