Question

how did christianity spread

Answer 1

Through evangelism, conquest, and missionary work. Answer We know that by the time of St Paul, Christianity had spread throughout much of the Near East and Greece, and at least as far as Rome. It may be that part of the reason for this spread was the spread of the diaspora Jews through the Roman Empire. However, Paul undoubtedly played a major part in this early expansion.
By the early fourth century, Christians formed a significant minority of the population of the Roman Empire, estimated at around ten per cent. When Constantine became the Roman Emperor, he gave Christianity state patronage and expended considerable state funds on a major program of church building and maintenance. When German tribes were conquered, Constantine required as part of the peace treaty that they convert to Christianity. Soon Christians were to be found throughout the empire. Before the end of the fourth century, Christianity was declared the state religion of the Roman Empire, and the public worship of the old gods was banned.
The next great wave of expansion came with the European conquests in the Americas, then Africa and the Pacific region. This took Christianity to all parts of the world. Answer Christians, primarily the Catholic Church, sent missionaries to minister in other countries.