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...
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