In I Thessalonians 3, Paul reveals that he was concerned about the Thessalonians. He feared that - because he had been rushed out of town during a time of persecution - the Thessalonians' faith would waver and diminish. Paul wondered, "How would this young church fare after its planters had suffered persecution and fled?"
To find out what happened to the Thessalonians following his abrupt exit, Paul dispatched Timothy to check on them. Fortunately, Timothy has just returned with an encouraging report. Rather than the persecution causing the church to stumble or fall away, they were aware that such things were bound to happen from Paul's warnings ahead of time. Therefore, Paul is filled with joy and looking forward to making a return visit!
The timing of this letter suggests that it was written very early, even sometime during Paul's second missionary journey or shortly thereafter. Several scholars believe that I Thessalonians may, in fact, be the earliest document (or one of the earliest) of the works that eventually went on to become the New Testament.