For also when we were with you we told you before that we are about to suffer tribulation, just as also happened and you know, 1 Thessalonians 3:4
After being chased out of Berea by the same Jews who stirred up trouble in Thessalonica, Paul was so concerned about the Thessalonians that he sent Timothy to establish and encourage their faith, while he remained alone in Athen. He was concerned that they would be shaken by the tribulation that was brought upon them by the Jews, even though he had warned them it was coming.
Tribulation is pressed from every side, greatly restricting movements or options. Because the Jews stirred up the city’s leaders to take surety from the saints in Thessalonica, they were able to impose pressure, severely limiting what the believers could do, especially regarding meetings and the spread of the gospel.
Knowing Satan would seize this opportunity to tempt them—solicit them to act in a wrong manner—Paul needed to know the state of their faith, which he discovered was very strong.