[Twosugars]

Originally Posted by LeatherTrumpet

this is a decent summary why it was popularised

https://www.quora.com/Why-did-the-pa...the-Jesus-sect

As others here have noted, Christianity had a number of advantages over its rivals. Firstly, it was open to anyone. Slaves, freedmen, barbarians and even women were welcome, regardless of their rank or position. In a society where hierarchy was supremely dominant, a religion where everyone was equal (at least when gathered in worship anyway) was actually very appealing. This inclusiveness also meant Christianity had some strength in numbers. One of the reasons we know so little about cults like that of Mithras and Cybele is that they were not just secretive mystery sects, but they were also very small. They were highly exclusive clubs, open only to selected, respectable and usually higher ranking people and by invitation only. This made them hard to compete with a sect that literally let anyone at all join.

Secondly, Christianity's message was simple and very attractive to many people as well as openly shared and proclaimed. No secret rituals in dark temples or underground grottos were required and the teaching weren't closely guarded layers of esoteric ideas. And the key message was about fellowship and forgiveness in this life and salvation and paradise in the next - things anyone could understand and appreciate.

Thirdly, the Christian emphasis on charity, fellowship and unselfishness was also very appealing. Christians were not only renowned for their care for one another but they also took seriously the idea of looking after others, especially the poor, the widowed and the orphaned. This idea of selfless giving and charity for its own sake was foreign to Greco-Roman culture, but appealing to many nonetheless. When Julian tried to revive state paganism he recognised that it was this unselfish charity that gave Christianity an edge and he tried to incorporate it in his new form of pagan faith, though without much success.

So when we put these social and theological advantages together with Constantine's adoption of his mother's faith after seeing a vision that he attributed to the Christian god, it makes sense that Christianity won out in the end.

Thanks for confirming what my quote said. In short, Christianity was attractive and therefore became popular. Despite the state persecution for three centuries it slowly spread and became fait accompli for the state