Why and How did Christianity become so popular?

Edward Gibbon, in his classical work, The History of the Decline and Fall of the Roman Empire, gives two reasons for its decline and fall: Christianity (internal) and Barbarism (external).  Edward Gibbon To understand the role of Christianity, one needs to go back to the two developments after the death (?) of Jesus. One was the takeover by James, the brother of Jesus, of the movement started by him; the other was the version of Christianity conceived and preached to the gentiles in the Greco-Roman world by the self-proclaimed apostle Paul as revealed to him by Jesus in ‘visions’ and through ‘disembodied voices’. During the period A D 66-70 the Jews revolted against Roman rule. This was brutally put down by the Romans and in the process they burned down and completely destroyed the Temple in Jerusalem. James and the followers of the Jesus movement were either killed or fled the massacre and the movement died a natural death. Had the Temple not been destroyed, perhaps Christianity would have been a continuation of Jesus movement and Paul would have been a foot-note in history. The fact remains that Paul’s version of Christianity remained and continued to flourish. What was a Jewish messianic sect became a Universal Salvation religion. The corner stone of Paul’s belief system was the divinity of Jesus. He saw Jesus as a saviour, a path for people to follow by which they might obtain eternal life. The followers of Paul who refused to accept Roman religious practices were killed. These were regarded as martyrs who were assured of heaven. (Virgins had 60 times the reward of ordinary Christians in heaven, but martyrs received rewards a hundredfold!). Christianity argued that ‘suffering is noble’ and offered a better world in future. Hence it became popular among the lower classes: the slaves, the labourers and the urban poor. Emperor Constantine (274–337). Emperor from 306 to 337. In A D 312 Constantine became Emperor and the fortunes of Christianity changed. Persecution was replaced by favour. He made Sunday observation compulsory. He started the practice of collecting relics to install in shrines. The spread of Paul’s version of Christianity was also helped by absorbing pagan practices where it was felt to serve its purpose. December 25, the feast day of god Mithras, became the date of Nativity; the original Sunday observance was conceived as a day of respect for the sun, not for Jesus. The terms ‘vicar’ and 'diocese' were borrowed from the Emperor’s administrative reforms. The original December 25th Virgin Birth There was another reason for the growth and spread of Christianity. People thought that religious solidarity would help the declining fortunes of the empire. This meant a crucial change in the reorganisation of society. This change resulted in the rise of the priesthood.