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What was the relationship between the Byzantine emperor and the patriarch of Constantinople?

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Hagia Sophia: mosaic — Byzantine Empire The Virgin Mary holding the Christ Child (centre), Justinian (left) holding a model of the Hagia Sophia, and Constantine (right) holding a model of the city of Constantinople; mosaic from the Hagia Sophia, 9th century.

The relationship between the Byzantine emperor and the patriarch of Constantinople is often characterized by the term caesaropapism, which suggests the emperor acted as head of the church. However, modern historians and official Byzantine texts describe the relationship as a "dyarchy"—a government of dual authority—or a "symphony" (symphōnia), where the emperor and patriarch functioned like the soul and body of a single organism.12

While the emperor acted as the protector of the church and managed its administrative affairs—often presiding over councils and influencing the appointment of patriarchs—he was not considered doctrinally infallible or invested with priestly powers. The church maintained authority over spiritual matters, including the preservation of Orthodox doctrine and order.123

Tensions frequently arose when imperial power attempted to override the church's spiritual freedom, such as during the Iconoclastic controversy or attempts at union with Rome. In such instances, the church often resisted, and strong patriarchs occasionally excommunicated emperors for uncanonical acts. Ultimately, the emperor was expected to be subject to the church's spiritual leadership as a "son of the church".123