In this short work we find Paul dealing with a continuing difficulty of the church:  the end times.  Someone has been misleading the Thessalonian church, telling them that the Day of the Lord had already passed (and you missed it). 

It is a frustrating work, for Paul does not lay out a concise overview of the end times, connecting his words with various prophecies.  He corrects the false doctrine; tells them to continue in the faith - and leaves it at that.  The subject is a touchy one even today; it is common for this "-ism" to sneer at that "-ism" when dealing with prophecy and end times.  The line between explanation and offense is often somewhat blurry.