Final answer:
Christianity in Europe spread through trade, military conquest, and political alliances, but chiefly through religious conversion efforts by Christian missionaries and the political ambitions of rulers who sought unity and strength.
Step-by-step explanation:
The spread of Christianity across Europe occurred through various methods, including trade routes, military conquest, and the use of political alliances. However, the most significant driver of this spread was the process of religious conversion. Christian missionaries displayed remarkable perseverance, and their success was bolstered by the political motivations of rulers to establish stronger ties with Christian polities. This blend of relentless mission work and political aspiration was instrumental in the conversion of many European peoples, including Germanic tribes and Slavic populations.
Moreover, Christianity was a proselytizing religion, with elite Christian thinkers dedicated to spreading religious knowledge and constructing a single Christian identity narrative. This effort was often supported by secular rulers, such as the Franks, who recognized the strategic value of fostering a unified religious landscape. The melding of religious fervor and military might aided in establishing a new, predominantly Christian medieval culture across Europe after the Roman Empire.
In conclusion, the spread of Christianity was a complex process influenced by missionaries, political leaders desiring unity and strength, and the pragmatism of negotiating with existing pagan traditions.