During the fifth and sixth centuries A.D. there arose on the Euphrates frontier, between the empires of Rome and Iran, a city girded with glittering gypsum walls. Within these walls stood a great church, a shrine for the relics of Saint Sergius, who was martyred there, at Rusafa, in the early fourth century. Around Rusafa stretched the "Barbarian Plain," inhabited by Rome's Arab allies, many of whom revered the saint. Elizabeth Key Fowden examines the rise of the cult of Sergius in late antiquity, drawing on literary accounts, inscriptions, archaeology, images, and the landscape itself to...