In Acts 13 and 14 we see the first missionary journey of Paul, traveling to Galatia (southern Turkey) and speaking to both Jews and Gentiles. The results of that journey lead to the Jerusalem council in Acts 15 and an endorsement of ministry to the non-Jews. Paul then returns to Turkey before moving on into Greece.
When Paul entered a new community, he usually went to the local synagogue to begin his preaching about the Jewish Messiah. The small assemblies (ekklesia) that developed in those communities grew out of the local synagogue and were, in many aspects, similar to the Jewish synagogues.
After Paul left a region, he stayed in contact with these assemblies, sometimes returning to visit, often writing letters to them about both administrative and theological matters.
In modern English terminology, we say "church" instead of "assembly" and Paul's letters to these churches are often called "epistles", a term which somehow sounds more formal to me. We should not forget that they were letters to small congregations, small "assemblies" of mixed Jew and Gentile believers.
One of Paul's earliest letters is his Epistle to the Galatians, a letter sent to numerous churches throughout southern Turkey. Other early letters, probably written within the timeline of Acts, include his two letters to Thessalonica and his two letters to the church in Corinth. We will look at those five early letters (I & II Thessalonians, Galatians and I & II Corinthians) after we finish the book of Acts.
Other letters by Paul include a letter to the church in Rome, written to believers Paul had not yet met.
That letter carefully lays out Paul's view of the global importance of the Jewish Messiah, Jesus. Other letters by Paul include letters to churches in Colossae, Philippi, and Ephesus. Additional letters preserved in the New Testament went to friends and fellow-ministers such as Titus and Timothy. (There is, of course, a good Wikipedia article on the letters of Paul.)
We will look at several of those letters after we conclude our study in Acts, beginning with I Thessalonians.