Why and how the religion of Islam spread relatively quickly in Iran in the early Muslim centuries is among topics that still attracts scholars in the field of history. To Know the religious and political conditions of Iran on the eve of the Muslim Arab arrival helps us better understand the factors and reasons for the Iranians' denial of their former religion and conversion to Islam. The contention of the present article is that in its final years, the Sasanian Empire was entangled in various domestic religious and political crises, a factor that was more instrumental in its fall than all other causes
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