The Roman Empire had remained, to a limited degree, multiethnic, multicultural, and multireligious, even after Christianity became its official religion. With the breakup of the Empire, the Western Church increasingly sought to assert its authority in the secular as well as in the spiritual realm. The society it envisioned was Christian in conformity with the doctrine laid down by the Roman Church. Within it the status of non-Chrisitians or unorthodox Christians became at best anomalous; at worst, these groups came to be threatened with persecution and even extinction.

In the eleventh century, Christian teaching about war changed. The religion that had emphasized passive suffering and martyrdom began a program of "holy wars," glorifying those who took up the cross not only as a badge of suffering but as a battle standard. To make peace among the barons who had been fighting one another, the Church enlisted them in crusades against the Moslems who had conquered the Middle East, North Africa, southern Spain, and much of Asia Minor. The crusaders were to be soldiers of God who fought with the promises of indulgence for sins and of salvation. Culminating in the capture of Jerusalem in 1099, with the slaughter of its Moslem and Jewish inhabitants, the First Crusade led to the establishment of Crusader kingdoms in the Middle East. These conquests were eventually eroded and the Christians driven out of their fortified cities. Jerusalem itself was recaptured by the armies of the great Arab general Saladin in 1187.

However, crusades were still being waged through the fourteenth century. The "worthiness" of Chaucer's Knight in "The General Prologue" to The Canterbury Tales is summed up by a long list of the crusades in which he took part (NAEL 8, 1.219-20, lines 47–68). For Chaucer's audience, ignorant of the sordidness of some of the campaigns waged in God's name, crusades still held an aura of heroism and glory, a spell they would continue to cast over the Western imagination for centuries.