Article Highlight: This study examines the role of Zoroastrian religious unity in Sasanian state formation and the consolidation of Ardashir I’s political authority, drawing on historical institutionalism and Weber’s theory of legitimacy. It argues that compiling the Avesta, centralizing fire temples, and organizing the priesthood fostered religious integration and traditional-religious legitimacy, thereby facilitating political centralization and Sasanian state institutionalization.

Introduction: The formation of the Sasanian state marked a major transformation in ancient Iranian political history, involving not only a dynastic change but also the reconstruction of political authority through the integration of religion and kingship. Zoroastrian religious unification played a significant role in establishing the ideological and institutional foundations of the new state. However, the mechanisms through which religious institutions contributed to political centralization and the consolidation of Ardashir I’s authority require further theoretical examination. This study therefore investigates the relationship between religious institutionalization, state formation, and political legitimacy.

Method: This research employs a descriptive-analytical method based on historical and library sources, drawing on historical institutionalism and Max Weber’s theory of legitimacy. Historical institutionalism conceptualizes state formation as a process of institution-building and political consolidation. Accordingly, the organization of the priesthood, the formalization of religious traditions, and the expansion of fire temples are examined as mechanisms of institutional integration. Weber’s theory of legitimacy provides a framework for analyzing how religious institutions and sacred traditions strengthened the traditional-religious legitimacy of Ardashir I’s kingship and facilitated the acceptance of the emerging political order.

Results and Discussion: The findings indicate that the unification of Zoroastrianism under Ardashir I was not merely a religious development but a central element of state-building. The establishment of an organized religious structure and the strengthening of the priesthood created an institutional network that supported the emerging royal administration. Through these institutions, the Sasanian state was able to overcome the fragmented political structure of the late Parthian period and establish a more centralized system of authority. Moreover, the recognition and organization of Zoroastrian traditions provided a shared ideological framework that strengthened social cohesion and legitimized royal power. Religious concepts such as divine charisma (xwarrah) and legitimate kingship linked the monarch’s authority to a sacred order and transformed kingship into an institution supported by religious values. From a Weberian perspective, these processes generated a form of traditional-religious legitimacy that strengthened the acceptance of Ardashir’s rule. The expansion of fire temples and the integration of religious practices with royal authority further spread the political ideology of the new state. Thus, Zoroastrianism functioned simultaneously as an institutional resource and a source of political legitimacy.

Conclusion: The study concludes that the unification of Zoroastrianism was a fundamental factor in the emergence and consolidation of the Sasanian state. Under Ardashir I, religion moved beyond the sphere of belief and became an essential component of political organization and state legitimacy. The interaction between religious institution-building and political legitimation enabled the new dynasty to transform a fragmented political order into a centralized state structure. Therefore, the success of Ardashir I should be understood not only as the result of military power or political circumstances but also as the outcome of creating durable religious institutions and a legitimate ideological foundation for royal authority.