There are two major factors that, in my opinion, contributed to the spread of Christianity within the Roman Empire.
- The fact that the Roman Empire existed. Because the Roman Empire existed, it tied together people in many different areas from what's now Turkey to Egypt to Northern Europe. People could travel and trade pretty freely within this area. Because it was so easy to move around (relatively speaking) the ideas of Christianity could be spread as Christians moved around the Empire. This would have been much harder if it had been split into a bunch of different countries.
- The conversion of Constantine. When he converted, Christianity became the state religion and that helped it spread tremendously.
Great question. Here are a few points to consider:
1. The most important factor for the growth of Christianity was that it wanted to grow. This factor might seem small, but this makes all the difference. Most religions in the Roman world did not seek out converts. The followers Epicurus and the Jews might be the only exceptions. Christianity, on the other hand, wanted to grow. Just look at Paul's missionary journeys.
2. Because Rome had such a great empire in terms of travel, any religion that wanted to spread could. In other words, Christianity came at the right time.
3. Christianity also had a great demonstration of power. A historian like Ramsey MacMullen (one of the great) even states that the ability of Christians to perform miracles helped its growth. Now, you might not believe in miracles, but the ancients certainly did!
4. Julian, the Roman emperor, also writes about Christian charity. This must have had an effect, especially during difficult times, like during plagues.
5. Some of the stories of heroic martyrdoms must have helped as well. Even a few stories can cause a buzz.
6. Some notable converts made a difference. Some were from the upper-classes. The most famous is obviously Constantine.
7. Finally, Christian apologetic literature should be included. This probably helped the educated class.
Great question. Here are a few points that Roman historians state:
1. Because Rome had such a great empire in terms of travel, any religion that wanted to spread could. In other words, Christianity came at the right time.
This was also the period of the Pax Ramona (Roman Peace).The Apostle Paul went on several missionary journeys and converted Romans to Christianity. The Roman Peace meant the Germanic invaders and other hordes were not a threat at that time. Internal decadence and moral decade were prevalent,but the onslaught of invasions came later. The example the Christians manifested, and doctrine they taught was alien to many of the pagans in Rome.They took note that this new religion brought a different perspective than the traditional pagan religions.Constantine's acceptance of Christianity as a legal religious systems certainly helped advance the spread.I personally believe Constantine did not understand all of the tenets that Christians stood for,but his 'conversion' as a person of power and influence made a positive difference in the history of Christian expansion.
THe main reason it spread throughout the empire is that if you didn't convert to Christianity you were killed.