Paul had only a few weeks in Thessalonica before the local religious leaders conspired to run him out of town. It left a lot of new believers with little training in their new faith but facing persecution for their beliefs. Paul’s letter, which was probably written in about 50 AD, was sent to encourage them:
1 Thessalonians 5:9-11 For God chose to save us through our Lord Jesus Christ, not to pour out his anger on us. Christ died for us so that, whether we are dead or alive when he returns, we can live with him forever. So encourage each other and build each other up, just as you are already doing.