Ardashir I

Ardashir began as a local ruler in Persis, the southwestern Iranian region that gave the Sasanian dynasty its name, reputedly connected to a family of hereditary custodians of a fire temple dedicated to the goddess Anahita at Istakhr. He expanded his territory through the 210s CE, defeated and killed the last Parthian Arsacid king, Artabanus V, at the Battle of Hormozdgan in 224, and was crowned King of Kings, ending nearly four centuries of Parthian rule and founding the dynasty that would govern Iran until the Arab conquest four centuries later.

Ardashir used explicitly religious imagery to stage this transition. A rock relief at Naqsh-e Rustam shows him receiving the ring of kingship directly from Ahura Mazda, mounted and facing the god as an equal in scale, a visual claim that his rule carried divine sanction rather than resting on conquest alone. His coinage carried Zoroastrian fire altar imagery from early in the reign, a break from the more varied Parthian precedent, and set a pattern his successors continued for the next four centuries.

Ardashir's early campaigns also brought him into conflict with regional rivals within Persis itself before he could turn against the Parthian crown, a sequence of local consolidation historians reconstruct chiefly from his own dynasty's later court histories and from Roman sources describing the new Sasanian state as a far more centralized and assertive neighbor than the Parthians had been.

Later Zoroastrian tradition, recorded centuries afterward in the Denkard and associated with a figure called Tansar, credits Ardashir's reign with beginning a deliberate program of religious consolidation, gathering scattered Avestan material, standardizing priestly practice, and asserting that political and religious order were a single project, summarized in a phrase historians gloss as one king, one religion. A letter attributed to Tansar defending forced religious unification on these grounds survives in a later Arabic and then Persian recension, and its precise relationship to events of Ardashir's actual reign is debated: Mary Boyce, among others, treats it as reflecting genuine early Sasanian religious policy dressed in later literary form, while other historians read it more as Sasanian era ideology projected backward to legitimize a later state of affairs.

What is not seriously disputed is the outcome: by the end of the Sasanian period, Zoroastrianism held a position as imperial state religion, with royal patronage of fire temples, an increasingly organized priestly hierarchy, and eventually the redaction of the Avesta into something closer to the scripture later Zoroastrians inherited, a process tradition traces to a beginning under Ardashir even where the details of his own personal role remain harder to verify than the inscriptions of a figure like Kartir make possible for the following century.