What historical factors explain how Christianity grew from a minority belief to the dominant religion of Western Europe?

The adoption of Christianity as a State Religion by the Roman Empire

Explanation:

Christianity originated in the Middle East and was mostly confined to the region with small missionary groups taking the religion to Southern Arabia and sometimes even as far as China.

However, the religion became dominant in Europe only after the might Roman Empire adopted it as the State religion. With the Empire's influence stretching over every European country, the religion overthrew paganism and has since shaped European identity.