In the centuries after Constantine, the most serious dispute within the Orthodox Church concerned icons — images or representations of God the Father, Jesus, and the saints in painting, bas-relief, or mosaic. Since the third century the church had allowed people to venerate icons. Although all prayer was to be directed to God the Father, Christian teaching held that icons representing the saints fostered reverence and that Jesus and the saints could most effectively plead a cause to God the Father. (For more about the role of saints, see “The Process of Conversion.”) Iconoclasts, those who favored the destruction of icons, argued that people were worshipping the image itself rather than what it signified. This, they claimed, constituted idolatry, a violation of one of the Ten Commandments, the religious and moral code found in Hebrew Scripture that was also sacred to Christians.

The result of this dispute was a terrible theological conflict, the iconoclastic controversy, that split the Byzantine world for a century. In 730 the emperor Leo III (r. 717–741) ordered the destruction of icons. The removal of these images from Byzantine churches provoked a violent reaction: entire provinces revolted, and the Byzantine Empire and the Roman papacy severed relations. Since Eastern monasteries were the fiercest defenders of icons, Leo’s son Constantine V (r. 741–775) seized their property, executed some of the monks, and forced other monks into the army. Theological disputes and civil disorder over the icons continued intermittently until 843, when the icons were restored.

The implications of the iconoclastic controversy extended far beyond strictly theological issues. Iconoclasm raised the question of the right of the emperor to intervene in religious disputes. Iconoclasm antagonized the pope and served to encourage him in his quest for an alliance with the Frankish monarchy (see “The Merovingians and Carolingians”). This further divided the two parts of Christendom, and in 1054 a theological disagreement led the pope in Rome and the patriarch of Constantinople to declare each other a heretic. The outcome was a continuing schism between the Roman Catholic and the Orthodox Churches.