Final answer:
Christianity spread across Europe through subtle integration of pagan practices and customs into the Christian faith. Cultural and religious differences emerged between European Catholics and Christians in the Middle East, reflecting varied practices under Muslim rule. By the 16th century, the Protestant Reformation began to challenge the Catholic Church's dominance in Western Europe.
Step-by-step explanation:
Spread of Christianity in Europe
Christianity in Europe experienced significant geographical and cultural spread from early times. The transformation and integration of pagan practices into Christian customs facilitated this spread. For instance, the days of the week in English have names derived from pagan gods integrated into Christian naming conventions, like 'Wednesday' for Odin, and 'Thursday' for Thor. This pragmatic approach to conversion allowed for a broad acceptance of Christianity's core truths, as opposed to strict doctrine understanding.
In places like the Middle East and the Byzantine Empire, existing Christian populations had different practices and sometimes conflicted with the newly arrived European Catholics. The establishment of Catholic authority in these regions was often met with resistance, as local communities had been shaped by centuries under Muslim rule, resulting in a Christianity that looked different from that in Europe. These local Christian groups spoke Arabic and adhered to unique cultural practices which distinguished them from the European Christians. Even languages such as Latin in the West and Greek in the East signified a deep division within the Christian dominions, as the Western church recognized the Pope's supremacy, and later on, inquisitions and military campaigns like the Crusades led to forced conversions.
The dominance of the Catholic Church in Western Europe was unchallenged up to the 16th century, marked by the onset of the Protestant Reformation which began to fracture the religious unity. The reformative movement was driven by the criticism of the Church's wealth, influence in political affairs, and the luxurious lifestyles of its clergy. As Europe expanded, so did Christianity, with Catholic missionaries accompanying colonial expeditions across the world. Despite these religious campaigns, internal and external challenges continuously reshaped the religious landscape.