How did European religion spread?

Answer 1

European religion is intertwined with European politics. Whoever rules has an influence on the people he/she rules. When Constantine became Christian, Christianity spread all over the Roman Empire. When European explorers and colonizers conquered other people, they spread their religion too. Missionaries still go to find people and convert them.

Answer 2

The Apostle Paul wrote to a Christian community in Rome around the middle of the first century, although we can not be sure how Christianity arrived there. It also appears that Paul intended to travel to Spain to preach Christianity, and Clement of Rome (1 Clement) seems to have believed that he did finish his life in Spain.

Until the time of Emperor Constantine, orthodox Christianity was very much a religion of the eastern Empire, apart from Rome and, to a lesser extent the city of Lyons. Adrian Hastings (A World History of Christianity, 150-550) says the impression given is that there can hardly have been more than a couple of hundred Christians in Lyons by 180 CE. However, Gnostics appear to have been more numerous in Gaul, as Irenaeus, bishop of Lyons at that time, complained of the followers of Valentinus in his diocese.

By the time of the Great Persecution, which lasted from 303 to 305 in the west, there were churches throughout much of Western Europe, although the proportion of Christians was still much less than ten per cent everywhere outside Rome. The western rulers protected their Christians from the harshness of the Persecution as much as they were able. On his accession to power, Constantine changed the religious environment forever. His Edict of Milan granted privileges to the Christian Church, although this was not applied to the benefit of the Gnostics. He gave the Church state patronage and political support, as well as beginning the long persecution of the pagan temples. By 380 CE, when Christianity became the official religion of the Roman Empire, Christians formed a substantial minority of the population of western Europe. The savagery of the persecution of pagans from 391 onwards, means that Christianity grew rapidly throughout the Roman Empire, until eventually the entire population could be assumed to be Christian.

It is not clear when or how Christianity first came to Britain, but archaeological evidence for Christian communities begins in the third and fourth centuries. During the late fifth century, Saint Patrick is thought to have brought Christianity to Ireland, which was never part of the Roman Empire.