The Apostle Paul ministered in the city of Thessalonica (present day northern Greece) during his second missionary journey and following his ministry at Philippi. The record of Paul's ministry in Thessalonica is found in Acts 17:1-10. His ministry was in the Jewish synagogue. The message was that Jesus is the Messiah, who had to suffer and rise from the dead. Some Jewish and Gentiles were converted. However, other Jews resisted and started a riot against Paul in the city. False charges were leveled at Paul and his companions before the city officials. This necessitated the exodus of Paul from Thessalonica.
The two Thessalonian letters, along with the Galatian letter, are probably the earliest that Paul wrote-around A.D. 51. In addition to general instructions on survival in the midst of persecution and on godly living, Paul wrote them about the coming of Christ. This is the only place in the New Testament where "the rapture" is indicated. It is also the first apostolic writing on "the coming" or the "parousia" of Jesus.
First Thessalonians is truly a helpful letter for the Christian believer.