Final answer:
The main religion in Europe during 800-1300 CE was Christianity, specifically Roman Catholicism, which was the dominant religious and social institution throughout the continent. Although Islam had a presence, particularly in the Iberian Peninsula, Christian forces were steadily reconquering these areas.
Step-by-step explanation:
The main religion in Europe during 800-1300 CE was Christianity, specifically in the form of Roman Catholicism. During this time, the Christian Church was the most significant religious and social institution in western and central Europe. It had a vast influence on the continent, from the governance and law to cultural practices and daily life. The power of the Church culminated in the year 800 when Pope Leo III crowned Charlemagne as the emperor of the Holy Roman Empire, solidifying the Church's influence over European politics and society.
Meanwhile, Islamic rule was present, especially in the Iberian Peninsula, but it was being pushed back by Christian forces. The Reconquista was a significant movement during this period, as Christian kingdoms in northern Spain fought to reconquer territories controlled by Muslim states. By the end of the 13th century, Christianity had retained and expanded its influence across Europe, with other religions such as Judaism and Islam becoming more marginal in these regions. Europe's near-uniform religious identity as Roman Catholic would only begin to fracture with the advent of the Protestant Reformation in the 16th century.