This letter was probably written shortly after First Thessalonians. The church at Thessalonica seemed to be doing pretty well except for one thing, manifested in two ways: they were so focused on the second coming of Christ that they weren’t maturing in other areas and were not taking care of their daily business.
It may be a little dramatic to say it this way, but in this letter, Paul is setting the stage for an “us versus them” mentality, where “us” are the believing Christians (especially the persevering true believers like the Thessalonians) and “them” are all others. Developing this type of mentality is a proven way for making people commit to a cause and keeping them committed. It is a simple way to develop distinguishing characteristics.
Unbelievers will be paid back trouble for the trouble they have caused, will be punished with everlasting destruction and kept from the presence of the Lord. This will happen when Jesus is revealed from heaven. Unbelievers are those who do not know God and do not obey the gospel of Jesus. (1:6-9)
Believers are those who endure through suffering. God will give them relief, and will be glorified through their lives. (1:5-12)
Paul reassures the Thessalonians that the Day of the Lord has not yet occurred, and it will not occur until certain things happened. Paul reminds them he had told them about these things before. These things may sound mysterious and incomprehensible to modern people because we do not know what he had previously told them in person. Paul must have thought this was sufficient information for them, though, so he moves on to other matters.
Among other things, he reminds the Thessalonians they must lead holy lives while they wait for the Day of the Lord. They must work to take care of themselves and set a good example for non-Christians, so that through their attractive lives they may bring others into the kingdom of God. That message was a big key to the growth of the church for the next two hundred years, and continues to be a good message today.