First Thessalonians written probably around 50AD is mostly considered to be Paul's first epistle. Some scholars believe Galatians was probably written in 48AD. How it mostly concerned public knowledge that Paul wrote Thessalonians first. The Epistle was written to the church in Thessalonica in modern-day Greece. The church at Thessalonica was one of the first Churches established early on in Paul’s ministry. The city of Thessalonica was established around 315 BC by Cassander, who named the City after his wife, Thessalonica the half-sister of Alexander the Great. They were mostly governed by the people and free from military or roman forces. As a result, it was more Greek than Roman socially. The population in Thessalonica is estimated to be about 200,000. A mixture of Greek, Roman, and Jewish communities. Religiously, Thessalonica was extremely pluralistic in its religious orientations with at least 25 gods represented in Heather worship. Due to the plurality of religions, there were many gentiles converts to Judaism. Paul reached Thessalonica during his ministry to Macedonia.
During this journey, Paul and Silas were beaten and later imprisoned (in Acts 17). Paul arrived in Thessalonica and began his ministry with jews first as is His custom. He visited the Synagogue and reasoned with them from the scriptures for three sabbaths. Due to many greek proselytes in Thessalonica, there were many greeks attending Synagogue and many of those greeks were persuaded by Paul’s message along with some Jews. As a result, The leader of the synagogues plotted to end Paul's ministry by seeking to destroy him. As a result, paul didn't get a chance to Establish a mature leadership in the Church of Thessalonica and so When he writes to them, he is mostly concerned about their faith firmly established in Christ and looking forward to the opportunity to visit them again.