If the conversation is about church growth, let’s pretend then that the Gospel actually has something to do with church growth. In that case, we should understand something about the Gospel and the beginning of the Jesus movement.
The word gospel has a bit of a history to it. In our church culture, we speak of gospel as “The Gospel.” It is code for the story of Israel fulfilled in Jesus Christ through his death, burial, and resurrection (cf. 1 Cor 15.1-5). But the word gospel, which means “good news,” has a history to it that is significant. In the ancient world a king would parade through the streets after winning a battle to tell the people of his victory by saying “Greetings…We are conquerors,” which was the king’s way of announcing salvation for the people. This practice was known as proclaiming the gospel or good news .
This was the practice of the Roman emperors, beginning with Caesar Augustus who regarded himself as a “son of a god” . From there on and with each new ascension to the throne, the Roman emperor would announce his gospel. So for the first followers of Jesus that formed this Jesus movement to proclaim another gospel, they were not simply making a theological affirmation. They were making a very risky political declaration that made them enemies of the state.
Beyond the political danger one assumed by confessing faith in Jesus Christ, the call of Jesus summoned believers to a strict moral lifestyle as well as the practice of self-sacrificial love that defied all social boundaries. Becoming a believer in the first-century was anything but easy. To become a believer meant assuming a radical counter-cultural lifestyle that denied lordship to Roman politics and rejected Roman culture as the way of life. This meant facing humiliation and persecution for the cause of Jesus Christ.
Yet as this Jesus movement swam upstream on a very strong downstream current, it exploded in growth. In no more than one-hundred years of time, this movement went from nothing to something so large that it was beginning to be regarded as a fourth human race .
It is common to hear about the social and geographical factors that made the timing right for the explosion of early Christianity. Such factors include the common language of Greek that was spoken throughout the empire, the fact that many people lived in urban cities and that there was good roads for traveling. This is all true but it fails to account for all the opposition that Christians faced as the Jesus movement began, which I have discussed above.
How then do we account for the massive church growth experienced? I am inclined to agree with N.T. Wright who says:
Why then did early Christianity spread? Because early Christians believed that what they had found to be true was true for the whole world. The impetus to mission sprang from the very heart of early Christian conviction… 
The Jesus movement took off with explosive growth first and foremost because those Jesus followers believed in Jesus. They believed that the only gospel was the story of Jesus’ death, burial, and resurrection.
- Scot McKnight, The King Jesus Gospel: The Original Good News Revisited (Grand Rapids: Zondervan, 2011), 50.
- Andrew J. Spallek, “The Origin and Meaning of Euangeliops in the Pauline Corpus” Concordia Theological Quarterly 57 (July 1993): 178.
- Everett Ferguson, Backgrounds of Early Christianity, 2d ed. (Grand Rapids: Eerdmans, 1993), 26.
- N. T. Wright, The New Testament and the People of God, Christian Origins and the Question of God, vol. 1 (Minneapolis: Fortress Press, 1992), 359.
- Ibid, 360.