At the opening of the sixth century, large segments of the Roman Empire had fallen to barbarian warlords. The Churches of Rome and Constantinople were locked in a schism rooted in different attitudes towards the decrees and definitions of the Fourth Ecumenical council held at Chalcedon in 451. The emperor Justinian (527-565) dreamed of reunifying and restoring the Empire; but to accomplish this he needed a unified Church. Before Justinian ascended...