This essay is concerned with the social and ideological accommodations of the East Syrian Church in Late Antiquity, by first incorporating, and later expanding a body of civil law. First under late Sasanian rule and then under its Islamic successor, East Syrian church leaders sought to promote and elaborate a distinct communal law that would solidify the primacy of ecclesiastical institutions and the communal fidelity of East Syrian Christians. An East Syrian ecclesiastical interest in worldly affairs was initially manifested in the contexts of an institutionally permeable Sasanian setting and of a rich matrix of Christian–Zoroastrian ties. Following the Islamic takeover, the expansion and subsequent codification of civil regulations in the eighth and ninth centuries were the products of a clearly defined community. Under early Islamic rule, the incorporation of civil regulations in ecclesiastical law speaks of the growing purview of the ecclesiastical judiciary and the institutional and theological competition that was posed by Islam.