No event during the first millennium was more unexpected, more calamitous, and more consequential for Christianity than the rise of Islam. Few irruptions in history have transformed societies so completely and irrevocably as did the conquest and expansion of the Arabs in the seventh century. And none came with greater swiftness. Within a decade three major cities in the Byzantine Christian Empire--Damascus in 635, Jerusalem in 638, and Alexandria in 641-fell to the invaders.
When reports began to circulate that something unusual was happening in the Arabian Peninsula, the Byzantines were preoccupied with the Sassanians in Persia who had sacked Jerusalem in 614 and made off with the relic of the True Cross. And in the West they were menaced by the Avars, a Mongolian people who had moved into the Balkans and were threatening Constantinople. Rumors about the emergence of a powerful leader among the Arabs in the distant Hijaz seemed no cause for alarm.
Even on the eve of the conquest of Jerusalem, when Arab armies had encircled the holy city and blocked the road to Bethlehem, the patriarch of Jerusalem, Sophronius, assured the faithful: "We will laugh at the demise of our enemies the Saracens [as Christians first called the Muslims] and in a short time see their destruction and complete ruin." Fourteen hundred years later the Muslims are still in Jerusalem, and with each passing decade Islam figures larger in the minds of Christians, penetrates more deeply into Christian societies, and, by its fixed and impermeable tenancy of a large part of the globe, circumscribes the practice of Christianity.
From the day Caliph Umar was met by the patriarch Sophronius in Jerusalem in the mid-seventh century, Christianity has found itself face to face with Islam. Though the circumstances have varied from place to place and century to century, Islam has always presented a challenge. Yet, in the course of a long history, during which Islam expanded all over the world, Christians, with the exception of those who lived in the Middle East in the early centuries of Muslim rule, have seldom taken Islam with the seriousness it deserves or recognized it for what it is--a religion in the biblical tradition in which piety is wedded to statecraft. A "complacent ignorance" (in the phrase of the modern scholar Lamin Sanneh) has prevailed, especially in the West.
Before the Muslim conquest, Christians could look back confidently on six hundred years of steady growth and expansion. By the year 300, churches were found in all the cities of the Roman Empire, from Spain and North Africa in the west to Egypt and Syria in the east, as well as in Asia Minor and the Balkans. In the fourth century the Armenians embraced the new religion, and on the eastern shore of the Black Sea the preaching of St. Nino led to the conversion of the Iberian royal house and the adoption of the Christian faith by the Georgians. To the south, Christianity reached Ethiopia in the fourth century and Nubia a century later. And there were Christian communities in Roman Gaul already in the second century and in Britain by the third century.
No less impressive was the spread of Christianity eastward. Accustomed to the colorful maps of Paul's missionary journeys printed in study Bibles, we are inclined to think that the initial expansion took place in the Mediterranean world. But in the vast region east of Jerusalem--Syria, Jordan, and Iraq, where Aramaic was the lingua franca--the majority of people had become Christian by the seventh century. The Christian gospel was carried even farther east to ancient Persia, and from there it traveled along the Silk Road into Central Asia: Uzbekistan, Tajikistan, Afghanistan. At some point during the first six centuries it reached the western shore of India and even China. In the seventh century, the global center of Christianity lay not in Europe but to the east of Jerusalem.
Though the …