The founding of the church in Thessalonica was mentioned in Acts 17:1-9 and in his letter to them, Paul encouraged the church to continue to walk in the way that pleases God (1 Thessalonians 4:1-8) and not grow weary (2 Thessalonians 3:13).
The First and Second Book of Thessalonians are amongst many other letters Paul wrote to the churches.
This is from the Story of God series.