Roman catholic church and the Islamic Empire were both responsible for both spreading the religion and unifying the beliefs of different regions under one single religious dogma.
Roman Catholic church still held a lot of power over many countries because of its status and the respect that all Europeans had for the representatives of god.
Similar can be said about those who would carry the world of Allah, spreading the religion around Asia and Africa, making it the gigantic religion it is today.
In the end, both of them ended up spreading religion in massive ways and creating many countries who were traditionally religious in blocks of a single religion associated with the spreading source.