<P> In Khorasan in the northeastern Iran, a 10th - century Iranian nobleman brought together four Zoroastrian priests to transcribe a Sassanid - era Middle Persian work titled Book of the Lord (Khwaday Namag) from Pahlavi script into Arabic script . This transcription, which remained in Middle Persian prose (an Arabic version, by al - Muqaffa, also exists), was completed in 957 and subsequently became the basis for Firdausi's Book of Kings . It became enormously popular among both Zoroastrians and Muslims, and also served to propagate the Sassanid justification for overthrowing the Arsacids (i.e., that the Sassanids had restored the faith to its "orthodox" form after the Hellenistic Arsacids had allowed Zoroastrianism to become corrupt). </P> <P> Among migrations were those to cities in (or on the margins of) the great salt deserts, in particular to Yazd and Kerman, which remain centers of Iranian Zoroastrianism to this day . Yazd became the seat of the Iranian high priests during Mongol Il - Khanate rule, when the "best hope for survival (for a non-Muslim) was to be inconspicuous ." Crucial to the present - day survival of Zoroastrianism was a migration from the northeastern Iranian town of "Sanjan in south - western Khorasan", to Gujarat, in western India . The descendants of that group are today known as the Parsis--"as the Gujaratis, from long tradition, called anyone from Iran"--who today represent the larger of the two groups of Zoroastrians . </P> <P> The struggle between Zoroastrianism and Islam declined in the 10th and 11th centuries . Local Iranian dynasties, "all vigorously Muslim," had emerged as largely independent vassals of the Caliphs . In the 16th century, in one of the early letters between Iranian Zoroastrians and their co-religionists in India, the priests of Yazd lamented that "no period (in human history), not even that of Alexander, had been more grievous or troublesome for the faithful than' this millennium of the demon of Wrath' ." </P> <P> Zoroastrianism has survived into the modern period, particularly in India, where it has been present since about the 9th century . </P>

The ancient religion of persia before zoroastrianism become dominant was most closely related to