political system in which the head of the state is also the head of the church and supreme judge in religious matters. The term is most frequently associated with the late Roman, or Byzantine, Empire (Byzantine Empire). Most modern historians recognize that the legal Byzantine texts speak of interdependence between the imperial and ecclesiastical structures rather than of a unilateral dependence of the latter; historians believe also that there was nothing in the Byzantine understanding of the Christian faith that would recognize the emperor as either doctrinally infallible or invested with priestly powers. Many historical instances of direct imperial pressure on the church ended in failure, e.g., the attempt of Zeno (474–491) and Anastasius I (491–518) in favour of monophysitism, and the efforts of Michael VIII Palaeologus (1259–82) in favour of union with Rome. John Chrysostom and most other authoritative Byzantine theologians denied imperial power over the church.

It was normal practice, however, for the Eastern Roman emperor to act as the protector of the universal church and as the manager of its administrative affairs. Eusebius of Caesarea called Constantine “the overseer of external” (as opposed to spiritual) church problems (episkopos tōn ektos). Emperors presided over councils, and their will was decisive in the appointment of patriarchs and in determining the territorial limits of their jurisdiction. Emperor Justinian I, in the preface to his Novella 6 (535), described the ideal relation between the sacerdotium and the imperium as a “symphony,” an essentially dynamic and moral interpretation of church-state relations that did allow numerous abuses but was hardly a submission of the church to the state.

Caesaropapism was more a reality in Russia, where the abuses of Ivan IV the Terrible went practically unopposed and where Peter the Great (Peter I) finally transformed the church into a department of the state (1721), although neither claimed to possess special doctrinal authority.

The concept of caesaropapism has also been applied in Western Christendom—for example, to the reign of Henry VIII in England, as well as to the principle cujus regio, ejus religio (“religion follows the sovereign”), which prevailed in Germany after the Reformation.