The integration of Sasanian Studies into the broader field of Late Antiquity is by now a well accepted proposition, one that is being applied with varying degrees of success in recent, more general studies on Sasanian Iran. However, in spite of the importance of synchrony as an attempt to evaluate Sasanian Iran in dialectics with other actors of the late antique world, a diachronic view, which assesses the impact of formative cultural forces throughout ancient Iranian history, as a corrective to the synchronic perspective, is of methodological importance. The institutionalization of religious groups, that is, the creation of the Exilarchate and the Church of Persia, within the Sasanian realm may benefit from comparisons with Achaemenid religious practices, and serve a a paradigm for diachrony.