After the decline of the Roman Empire emerged three civilizations based on religion: Byzantium, Islam, and Latin Christendom. Byzantium encompassed the eastern half of the old Empire, the provinces that had always been more populous, prosperous, and advanced. Built upon a combination of Christianity and Greco-Roman culture, Byzantium clashed with the Roman church politically and theologically; the two ultimately split in 1054. Under Justinian Byzantium reached its political height, reconquering portions of the old western Empire. Byzantium soon lost these territories, however, as it turned its attention to attacks by the Persians and tribes invading from the Balkans. Successive conflicts with Islam and Latin Christendom gradually eroded Byzantium. It finally collapsed in 1453 when the Ottoman Turks captured Constantinople. Byzantium left an important legacy to the West, including the Corpus Juris Civilis codified under Justinian, the preservation of Greek learning, and the advancement of Greek Christianity into eastern Europe.

Islam was founded by Muhammad who proclaimed it the fulfillment of Judaism and Christianity. Preaching the Five Pillars of Faith and other rules set down in the Koran, Muhammad converted the pagan Arab tribes and suppressed his opponents. United around the new faith, these tribes launched a holy war of conquest that, by 732, gained them an empire that stretched from Spain to eastern Iran. This state was a theocracy governed according to Koranic law in which Jews and Christians enjoyed special rights as "peoples of the book." In this religious civilization dissent took the form of sectarian struggle, the most important of which was the split between Sunnis and Shi'ites. During the golden age of Islam, Muslim intellectuals (e.g. Rhazes, Avicenna, and Averröes) both preserved and amplified the Greek tradition, passing to the West invaluable works in medicine, science, and philosophy. Attacks by Turks, Mongols, and Europeans fragmented the Arab empire into states unified religiously though not politically. The Ottoman Turks ultimately emerged as the greatest Islamic power, building an empire that, by the mid sixteenth century, extended far into south-eastern Europe. Although able administrators, the Ottomans never recovered the creative vitality of medieval Islam.

Latin Christendom evolved more slowly than Byzantium or Islam, but ultimately developed into a unique civilization combining Christian, Greco-Roman, and Germanic elements. After shattering the western Empire, the Germanic tribes established kingdoms which they ruled according to their tribal customs. Consequently, urban life and long-distance commerce severely declined. Intellectual life decayed as well, such that scholars including Boethius, Cassiodorus, and Isidore of Seville preserved the Greek tradition rather than added to it creatively. The church became the principal force of civilization and unity. Monks and nuns copied classical texts, and monasteries became centers for converting pagan peoples. Under Gregory the Great, the church cultivated Roman administration, organized the work of monks nuns, and priests throughout Europe, and sought to protect its worldly position. It achieved the last of these goals through its long alliance with the Franks, whose unstable kingdom was finally unified by Charlemagne. Endorsed by the papacy, Charlemagne's rule and the Carolingian Renaissance he fostered advanced the synthesis of Christian, classical, and Germanic culture. That process slowed when, after his death, his empire disintegrated, and Europe endured devastating attacks by Muslims, Magyars, and Norse Vikings. During this period centralized authority gave way to feudalism that functioned through customary law, traditional obligation, and manorial economics. This system endured until the High Middle Ages, when urban economies revived and kings began to reassert their power.