There are, sometimes ironically, certain traits the Roman Empire possessed which contributed greatly to the growth of Christianity. The Roman Empire, most importantly, united a great variety and cultures and peoples into what in effect became one nation; the resulting unity and communication between so many peoples eased the spread of Christianity all over known world. The very cruelty of the Roman emperors assisted in spreading the Christian faith; not despite, but aided by their persecution Christianity grew rapidly. Finally, the oppressed and downtrodden nature of the plebeians made them very susceptible to and desirous of the Christian message.
The unity and ease of travel between many different peoples which the Roman Empire provided greatly aided in the sharing of the Christian message. The Roman Empire is the prototype for domination; at its height many hundreds of peoples were under its control. Roads led from Rome to every part of its dominion. Travel was made, if not easy, at least not impossible. This ease of communication made it possible for disciples to spread the Christian message all over the ancient world. If the Roman Empire had not existed, missionaries would have found it extremely difficult to cross the borders of very different and often warring nations without finding themselves accused and punished as an enemy, before they had a chance to spread the good news.
The very cruelty and hate of Christianity that various Roman emperors exhibited actually worked in favor of that religion. As it is said, "The blood of martyrs is the seed of the Church," for in many ways, persecution was beneficial to the new religion. Persecution gave the Christians a chance to flex their spiritual muscles, which many did, to great effect. The miracles that took place during these martyrdoms converted many of the pagans, which in turn led to more martyrdoms. Persecution served, essentially, to strengthen the faith of those already Christians, and to convert those who were not. By the very cruelties with which the emperors attempted to crush the young religion they actually aided it.
The oppressed, poor, miserable condition of the plebeians and the people in the countries Roma had conquered was also a circumstance which served Christianity. Jesus loved the poor; he told his Apostles that the poor were more open to his message. There were plenty of poor in Rome and in the countries subservient to her; in fact, most of the city and empire was destitute. This made average person living in the Empire more open and susceptible to Christ's message, a message which gave them hope of a better life hereafter, despite present poverty and misery. If the lower class had been well-treated and prosperous, they would probably have been less willing to believe in the next life; since this world had made them happy, what did they have to do with the next?
There were many factors which contributed to the spread of Christianity, but some of the most prominent and intriguing can be found in certain characteristics of Rome. The grand dimensions of the Empire, the fact that it encompassed so many different peoples, greatly aided in the spread of Christianity by enabling travel between such a great variety of nations over such a great area. The very cruelty of the Roman empires enlivened the faith of those already converted and converted yet more. And lastly, the oppressed nature of the subjects of the Roman Empire made the message of Christ welcome to these lower classes. God has a purpose behind everything, a plan guiding even the rise of the Roman Empire, writes St. Augustine. It seems to me that God allowed the rise of the Roman Empire so it could become the vehicle by which Christ's message would be spread throughout the ancient world.