Sasanian Empire
Tansar and his justification for Ardashir I's rebellion

From the very beginning of Sassanid rule in 224, an orthodox Pars -oriented Zoroastrian tradition would play an important part in influencing and lending legitimization to the state until its collapse in the mid-7th century. After Ardashir I had deposed the last Parthian King, Artabanus IV, he sought the aid of Tansar, a herbad (high priest) of the Iranian Zoroastrians to aid him in acquiring legitimization for the new dynasty. This Tansar did by writing to the nominal and vassal kings in different regions of Iran to accept Ardashir I as their new King, most notably in the Letter of Tansar, which was addressed to Gushnasp, the vassal king of Tabarestan. Gushnasp had accused Ardashir I of having forsaken tradition by usurping the throne, and that while his actions "may have been good for the World" they were "bad for the faith". Tansar refuted these charges in his letter to Gushnasp by proclaiming that not all of the old ways had been good, and that Ardashir was more virtuous than his predecessors. The Letter of Tansar included some attacks on the religious practices and orientation of the Parthians, who did not follow an orthodox Zoroastrian tradition but rather a heterodox one, and so attempted to justify Ardashir's rebellion against them by arguing that Zoroastrianism had 'decayed' after Alexander's invasion, a decay which had continued under the Parthians and so needed to be 'restored'.

Tansar would later help to oversee the formation of a single 'Zoroastrian church' under the control of the Persian magi, alongside the establishment of a single set of Avestan texts, which he himself approved and authorised.
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