A very good reason of the quick expansion of the earch church is the perperct message and its universality. Examples can be seen below.

Acts 13:47-48Paul and Barnabas' preach: For this is what the Lord has commanded us: "'I have made you a light for the Gentiles, that you may bring salvation to the ends of the earth.'"When the Gentiles heard this, they were glad and honored the word of the Lord; and all who were appointed for eternal life believed.(The message was received by Gentile)

15:8Peter's preach:God, who knows the heart, showed that he accepted them by giving the Holy Spirit to them, just as he did to us.(The message about Holy Spirit delighted people because it would reach universally without biase.)

15:19-20 Jame's preach: It is my judgment, therefore, that we should not make it difficult for the Gentiles who are turning to God.Instead we should write to them, telling them to abstain from food polluted by idols, from sexual immorality, from the meat of strangled animals and from blood.(The gospel is made easier for gentiles to obey)

16:5-6 As they traveled from town to town, they delivered the decisions reached by the apostles and elders in Jerusalem for the people to obey.So the churches were strengthened in the faith and grew daily in numbers.(The message is obeyable and the number of people who got salvation increased.)

17:16 So he reasoned in the synagogue with the Jews and the God-fearing Greeks, as well as in the marketplace day by day with those who happened to be there.(Paul reached the Greek, targeting people who are idol-worshiping and considering themselves philosophers.)

According to the examples from Acts, the Gospel message is widely-inclusive and made for people to obey and the most importantly, targets for both Jews and Gentiles including Greeks, Samaritan and some others.