Evertt Ferguson, writing on the expansion of the Christian Church in the first three centuries, notes that “by the end of the third century Christians made up a sizeable minority of the population of the Roman Empire” (Ferguson, Church History, From Christ to Pre-Reformation, 174).

Historians point to a number of factors that led to the expansion of Jesus’ people. Ferguson lists several and I summarize them here.

Externally:

1. Judaism provided a “base of operations” from which Christians could proclaim the gospel in the Roman Empire.

2. The presence of Hellenism (i.e. Greek culture) provided a common language and an ideological atmosphere that aided in Christian expansion.

3. Rome had established a relatively peaceful world in which followers of Jesus could move and minister.

Internally:

1. Christian conviction was often attractive and motivating.

2. The inclusive nature of the gospel.

3. The love for each other and meeting of practical needs.

4. Self-governing congregations that were tightly linked to one another [what Jonathan Leeman might call, interdependence].

5. A sense of deep community (i.e. fellowship).

6. Intellectual respectability matched with devout religious practice.

In addition to the things above, Ferguson also notes the message of hope and the egalitarian spirit (i.e all people equal at the cross) were attractive. Of course, Fergusson would agree that theologically we understand the growth of the church is always because a sovereign God moves powerfully by the Spirit through the gospel of Jesus Christ.

For more, see Everett Ferguson, Church History: From Christ to Pre-Reformation.