Peter's Rock in Mohammed's Flood, from St. Gregory the Great to St. Leo III

In this rigorous 19th-century study, T.W. Allies traces the fate of Western Christianity across two pivotal centuries, from the death of Gregory the Great in 604 to the reign of Leo III in the early 800s. This was the age when the Mediterranean world fractured: Byzantine power receded, the Islamic conquests swept across the Levant, North Africa, and Spain, and the papacy stood increasingly alone as the defender of Catholic orthodoxy. Allies writes as a committed Anglican making peace with Roman Catholicism, and his narrative pulses with a specific theological conviction, that Peter's See held firm against the 'flood' of Islam precisely because divine providence had appointed it the rock upon which Christ's church would stand. The book examines the complex dance between popes and Byzantine emperors, the iconoclast controversy, Charlemagne's coronation, and the slow emergence of Christendom as a Western political and spiritual entity. For readers interested in how the medieval world took shape, or how the papacy became the institution that would dominate European politics for a millennium, Allies provides a richly detailed, argumentatively bold portrait of the church navigating existential crisis.