But even more importantly, for the Christianisation of the Empire, and the regularisation of Christianity, was the conversion of Constantine the Great(c.312AD) as a result of a victory in battle, using the standard of the Cross. After this, Christianity was secure.
After the resurrection of Jesus, the group of disciples was very small. It included a few dozen people led by St. Peter and other Apostles. But three and a half centuries later, Christianity became the official religion of the Roman Empire. According to historians, a key factor in all this was the way early Christians lived.