Christianity rose up from being a minority religion to being the majority! It was viewed with suspicion at first, but it grew and important people caught onto it. Constantine was the first to make it the religion of Rome and he helped establish the Nicean council who made the Nicene Creed. Other rulers followed suit and converted conquered peoples. The Byzantines made their empire exclusively Christian. Medieval people forcefully converted a lot of people, they christianized pagans in Europe. Christianity had “benefits” for believers!
Christianity probably appealed to people in several ways. First of all, it did have a very high moral standard that it set forth…. Of course some philosophical sects and groups would also put forth rather similar ways of life for their practitioners. Christianity had an institution that provided material benefits but also had a whole sacramental system that offered to its practitioners, supposedly, repentance from sins and overcoming sin and overcoming death… As the church developed, it allowed for different degrees of Christian devotion. So, that if you wanted to give yourself up to a highly ascetic life and renounce practically everything, you would be much glorified for doing that, but you could be married and have a position in worldly life and have a family, career and so on and that was all right, too. So, Christianity could adjust itself to different types of people, just as it could adjust itself to the highest class of intellectuals but also adjust itself to common people whom the church writers always remind the theologians that Christ died for the lowly, as well as, for the educated. (Frontline From Jesus to Christ PBS)
Christianity became very powerful and was spread all over soon! There are 2.18 billion Christians today! About 1/3 of the world population!