New Testament (NT) scholars tell us that Paul, in the Book of Acts, had a change of strategy. In the early years, he would always go to a synagogue first to preach the Word and start a missionary work (Acts 13:5, 14; and 14:1). Luke even says that "it was Paul's custom" to start ministry in the synagogues (17:2). Later, these NT experts tell us that Paul, during his Third Missionary Journey, started to preach the gospel in places other than synagogues. He used the School of Tyrannus, a public forum for gentiles, as a location for doing missionary work and public evangelism (Acts 19:9). These experts and scholars say Paul had a change of strategy. I agree. And I would even go further and say that Paul also had a change of heart, or renewed view of the missionary work.
In the beginning, Paul was always excited about doing ministry. Right after his conversion, Luke tells us that Paul "immediately began to proclaim Jesus in the synagogues" (Acts 9:20 NASB). He was very bold. He always had his eye on the gentile horizon. He knew, even early on, that his preaching would bring him to a missionary work among the gentiles and non-Jews (13:47). I think, however, that Paul's vision for the gentiles at this point was located in the political structures of the synagogues. Simply said, he felt that missionary work among the Romans, Greeks, and other peoples must occur within the confines of the synagogue meeting place.
During the Second Missionary Journey, we see Paul starting to venture outside the synagogue and using a "place of prayer" as a location for missionary work. It is here that Paul encountered Lydia, the first convert in Europe. But we must mention at this point that this change of location was unintentional. Paul, Luke, and their companions were originally looking for a "place of prayer," possibly a religious meeting place similar to a synagogue (Acts 16:13). They, however, found Lydia by this riverside location and the rest is history, the story of the first missionary work in Europe. This approach of using the synagogues as a platform for missionary preaching continued even until the Third Missionary Journey. (Compare Acts 17:1-3, 17; 18:4,19; and 19:8.) Paul sees the synagogue as the place where cross-cultural ministry to the non-Jews and gentile God-fearers must occur. This is the strategy for missionary work. He could not think of any other way.
It is during his trip to Rome, that Paul realizes cross-cultural ministry among the Romans, Greeks, Africans, and Asians, is going to happen outside the confines of the Jewish synagogues. God reveals to Paul that witnessing in Rome means without the comforts of the synagogues. (Compare Acts 23:11 and 27:23-24.) Paul has a change of heart.
We see Paul's change of heart more clearly when we look at chapter twenty-eight. After being shipwrecked and resting in the island of Malta, Paul, his companions, the Roman soldiers, and sailors of the ship find themselves the recipients of the islander's hospitality. There were many occasions where Paul could have started preaching. But he did not. He waited for God's visitation. When the islanders saw a miracle and thought Paul was a god, Paul did not come out preaching and "doing" ministry. He waited for God. He waited for three days. God came when the father of the leading man of the island was healed from an affliction. God visited through Paul's prayers for healing. Many people received God's blessings thereafter. (See Acts 28:1-10)
As I mentioned above, Paul had a change of missionary strategy. He moved from preaching in the Jewish synagogues to Roman-Greek lecture halls, such as the School of Tyrannus. He also had a change of heart. He started with a great enthusiasm for public preaching and debate proving that Jesus is the Son of God (Acts 9:20-22; 17:1-4; and 18:5-6). He continued on towards a more contemplative ministry--waiting on God to act first before moving forward.
There is a change of strategy. There is also a change of heart. But more importantly is that there is a change of view of the missionary work. God's works among the gentiles and non-Jews have moved from the familiarity of the Jewish synagogues to the obscurity of Roman and Greek public places. Paul declares that the "salvation of God has been sent to the gentiles" (Acts 28:28 NASB). God is the missionary God who will bring gentiles to salvation regardless of the presence or absence of the Jewish synagogue. Paul concludes with this declaration and rightly states that the gentiles will listen (v. 28). The missionary work among the Romans, Greeks, Africans, and Asians will prosper because God is the one leading it. All we need to do is to wait on God to act first, and then we can move on to cross many different cultures and foreign lands.