In his book Introduction to World Christian History, Derek Cooper (2016) explores the underappreciated history of the church beyond Europe and North America. At the turn of the first millennium, Christianity began to fade in the eastern and southern Mediterranean world while it simultaneously grew in its western and northern parts (Cooper, 2016, p. 66). The crowning of Charlemagne by Pope Leo III in 800 marked was a watershed moment in the fusion of the Germanic and Roman cultures through a common faith and an imperialistic mindset. The other great part of European Christianity was the Byzantine Empire which had evangelized Belarus, Bulgaria, Greece, Malta, Macedonia, Moldavia, Serbia, Romania, and Russia. But as the Byzantine Empire declined it became vulnerable to the Muslim threat. Western Catholicism in contrast would expand into Africa, Asia, and eventually the Americas by the 15th century (p. 66). 

Eastern Europe was either Catholic or Orthodox based on the link to the Holy Roman or Byzantine empires. Further east in Belarus, Bulgaria, Moldova, Romania, Russia and Ukraine was mostly Orthodox while the Czech Republic, Hungary, Poland and Slovakia were Catholic (p. 67). Internal missionary battles existed between Catholics and Orthodox along the fault lines, particularly intense in Bohemia (p. 68). Although this competition could be seen as undermining Christianity in Europe, I believe it contributed to the intensification of its influence. 

Northern Europe was the last part of the continent to be evangelized, except for the British Isles (p. 69). The last areas to convert were Scandinavia and the Baltics, and in the latter all nations had adopted Christianity as the state religion by the 14th century. The conversion of Denmark, Norway, and Sweden came through the conversion of their kings who then instituted religious authoritarianism opposing religious diversity. Thus, in the North the influence of Christianity flowed from its connection to political power (p. 69-70). 

Christianity had been in Southern Europe since the 1st century. In the Southwest Catholicism was supreme, holding the sword in the left hand and Scripture in the right uniting conquest and evangelization (p. 71). The Franks and Romans wed their powers under Charlemagne who, along with his successors, employed pious but violent imperial missionary efforts. From the 8-14th centuries Many Balkan kingdoms adopted Eastern Orthodox as official religion even as Byzantine Empire collapsed. In Serbia, the presence of Catholics and orthodox were grounds for future conflicts, eventually becoming predominantly Serbian Orthodox. The Balkans today are roughly one third Orthodox, Catholic, and Muslim. Serbian association with Orthodox Christianity made them not convert as willingly as groups with weaker national churches, such as Bosnians. Again, here in the South the evidence of the power of church survival and its connection to political power is evident (p. 71-73). 

The battle between Christianity and Islam continued in Western Europe decades after the Battle of Tours. The Carolingian Dynasty was blessed by the papacy, thus guaranteeing the survival of the Franks. This alliance became the basis of the Holy Roman Empire and led to crusades in Holy Land and Europe. But later, struggles would arise in the West between the papacy and monarchs (p. 74). Medieval monastic reform movements such as the Cluniacs and Cistercians spread from France across Europe playing a central role in education and evangelization. Fringe Christian protest groups emerged which were either suppressed or eventually approved by Rome, such as Dominican and Franciscan orders that invigorating the church. Urbanization and the decline of feudalism led to the creation of secular Christian universities derived from the cathedral schools Charlemagne had established. Scholasticism emerged in Paris with figures like Aquinas and other “spell-binding professors”, which some thought was “rotten” but eventually developed into the liberal arts education. The Black Death caused “widespread religious anxiety and despair” and the papal controversies contributed to the Hundred Years War between England and France in the 14th century (p. 74-77).

Thus, despite internal and external challenges, Europe became center of Christianity after second millennium, congealing by the 15th century into respective religious borders: Catholic Southwest, Northwest, and West versus Orthodox in the Southeast and Northeast (p. 77). 

References

Cooper, D. (2016). Introduction to World Christian History. IVP Academic.