Let us also remember the way in which the Roman Empire, through its road building activities, helped the movement of people and therefore of ideas and new beliefs. The system of roads that proved so vital to the Roman Empire and allowed it to control a vast number of different and geographically distant territories also had an impact when it came to the dispersion of new ideas and trade.
Roman citizens were incredibly class conscious; a class system artificially based on Latin grammar. Those who only used the common Latin of the masses were the "vulgar" people, vulgar meaning "common." Christianity drew no distinction between classes; in fact all were considered equal in the eyes of God. Husbands were taught to honor their wives; Jesus often spoke of the blessedness of the poor; and of ridding ones self of all earthly possessions. This belief system created a sense of equality and a heavenly reward beyond earth that made even the lowest of the low feel worthy. They were reminded of the humble birth of Christ himself; and how his message had been spread by simple fishermen. These factors made it enormously appealling to the poor who embraced Christianity with enthusiasm.
I think that one such factor was the fact that there was a big split between the rich and the poor. The poor were more likely to become Christian because that faith promised them a better life in the next world to make up for the fact that they were not getting ahead in this world.
You could also say that the fact that Latin and Greek were common tongues for everyone in the empire made it easier for the new faith to spread.