The expansion of the Roman Empire persecuted Christianity in the early stage and supported Christianity in the later stage. In view of the great influence of Christianity in Europe in the future, I think the biggest influence should be to promote the establishment of Christianity.

In 800 AD, Pope Leo III crowned Charles at St Peter's Cathedral in Rome, honoring him as the "Emperor of Rome", which was the final completion of Frank's alliance with the Pope. At this time, Charles Empire reached the Elbe River and Danube River in the east, the Atlantic Ocean in the west, the North Sea in the north, and the Pyrenees and Italy in the south. Charlemagne became the master of western Europe. Charlemagne unified Western Europe and was known as the "father of Western Europe". Since then, Western Europe has formed a unified Christian cultural area, which has had a far-reaching impact on the Crusades in Western Europe and even the establishment of the European Union.

Charlemagne was a Germanic king, surrounded by Germanic knights and nobles, but he hired priests and classical scholars. Charlemagne gathered scholars from all over Europe in the court. He asked churches and monasteries to set up schools with seven subjects, namely grammar, rhetoric, logic, arithmetic, geometry and astronomical music, in order to preserve and spread the basic knowledge of classical-Christian culture. In this way, in the eighth century, the three elements of medieval western European civilization: classical, Christian and Germanic cultural elements were first merged together to some extent. Although Charlemagne Empire is basically Germanic, it can absorb a lot of classical and Christian cultures. However, compared with the Byzantine Empire and the Arab Empire in the same period, the Western Europeans under Charlemagne were still semi-civilized farmers.