ALTHOUGH WESTERNERS did not set out to explore the world until the fifteenth century, their beliefs had long since penetrated far and wide. When Constantine the Great and his colleague Licinius had declared Christianity a lawful religion in the Roman Empire in AD 313, they ended almost three centuries of sporadic but sometimes severe persecution. There were many different Christian sects in the fourth century, but the largest and best organised called itself the Catholic (or universal) church and in 392 Theodosius I made Catholic Christianity the official religion of the Empire. The other sects had died out by c.700 and almost all the churches of the medieval world traced their descent from the Catholic church of the fourth century. They accepted the same Biblical books as canonical; their public worship centred on the eucharist, and authority in all of them was vested in bishops. Medieval Christianity in all its forms was deeply influenced by monasticism, a practice that had spread from fourth-century Egypt to all parts of the Christian world, and men and women who lived as religious solitaries were held in particularly high esteem.
The Catholic church, which worshipped in Latin and acknowledged the pope as its senior bishop, was the only institution that survived the collapse of Roman power in the western provinces during the fifth century and the formation of independent kingdoms there by Germanic settlers. By the seventh century all those rulers had been converted to Catholicism, which had also spread beyond the former imperial frontiers to the Celtic lands in Scotland and Ireland. Medieval Western Europe may have been politically fragmented but it remained united in a shared religious faith.
Catholic Europe proved resilient to attacks from new enemies in the years 800-1000--the Vikings from the north, the Magyars from the east and the Muslims of North Africa from the south. Partly as a result of intermarriage between the invaders and Western Christians, the Catholic religion spread throughout Scandinavia and also to the new lands which the Vikings had discovered and settled in the north Atlantic, notably Iceland and Greenland, where a bishopric was established in 1112. Similarly, the Magyars, together with the other peoples of central Europe, such as the Bohemians and the Poles, were converted to Catholicism by c.1000. Sicily, Iberia and the Balearic islands were recaptured from the Muslims in a series of wars supported by the Papacy, which began in the eleventh century but only ended when Granada fell to the Catholic kings of Spain in 1492. Although there were Jewish communities in some cities and groups of Muslims in some southern frontier regions, by 1050 the vast majority of the inhabitants of western Europe were members of the Catholic Church. Small dissenting movements developed during the eleventh century and a tradition of dissent lived on throughout the rest of the Middle Ages, but its impact was limited except in some areas such as thirteenth-century Languedoc and fifteenth-century Bohemia.
In the early medieval centuries the Western church had been the custodian of literacy in a barbarian world and it inevitably became involved in the work of secular government, since rulers relied on the clergy to draft laws and keep records. The Church worshipped in Latin and preserved the Classical tradition of learning in some of its monastic and cathedral schools. Western Catholic civilisation reached maturity in the twelfth and thirteenth centuries, when its growth of scholarship found institutional expression in the emergence of universities where students were trained to argue in terms of Aristotelian logic. In time this led to the reformulation of Christian doctrine by theologians such as St Thomas Aquinas, who sought to demonstrate that there was no necessary conflict between human reason and divine revelation.
The foundation of the Dominican and Franciscan Orders of friars in the thirteenth century transformed the spiritual life of the Western Church. …