It’s been often heard that, as opposed to the Achaemenids and the Parthians, the Sasanians promoted a Zoroastrian religious “orthodoxy.” Yet, efforts by some of the most eminent scholars of the field to define the nature of this orthodoxy, the mechanisms for establishing it, and its distinguishing characters from heterodoxy, has been proven unsuccessful! 
Now, it has been generally accepted that some of the canonical Middle Persian Zoroastrian writings, including Dēnkard, Dādestān-ī Dēnig, Mātakān-ī Hazār Dātistān, Ardāvirāz Nāmag, Kārnāmag-ī Ardaxšēr-ī Pāpakan, Vizīdagīhā-ī Zādisparam and Letters of Manučihr, as well as some minor texts (such as those of Adurbād-ī Farrōbaγān: Rivāyat, Andarz and Mātigān- ī Gujastak Abālish), had been finally composed and written down in south-western Iran and specifically in the Fārs province, mostly during the 9th-10th century CE.
In this paper, I will argue that the often neglected local and regional aspects of these texts, have had a huge impact on shaping our understanding of what the Zoroastrian orthodoxy is. Additionally, there will be another argument against the current approach by mainstream scholars in ignoring those features of Iranian pre-Islamic religions which deviate from our definition of the orthodoxy. We have to reconsider taking the claim of the 9th-10th centuries’ mobeds of Pārs that Iranians during the Sasanian period were practicing their version of orthodoxy, at face value.
By re-examining and contextualizing some of the mentioned texts, as well as taking new archeological evidence and some literary sources (such as Shāhnāmeh) into consideration, I will demonstrate that, this perspective is valid, and may even provide us with new grounds for investigation. Hopefully, this may create a more accurate picture of the religious life of Iranians during the Sasanian – early Islamic period.
1-For example, look at: Zaehner, R. C., 1955, Zurvan: A Zoroastrian Dilemma, Oxford; Shaked, Shaul, 1994, Dualism in Transformation: Varieties of Religion in Sasanian Iran, London: School for African and Oriental Studies; De Jong, Albert, 1998, Traditions of the Magi: Zoroastrianism in Greek and Latin Literature (Religions in the Graeco-Roman World), Leiden; and Boyce, Mary, 1996, “On the Orthodoxy of Sasanian Zoroastrianism.” Bulletin for the School of Oriental and African Studies 59: 11-28.