This is evidence of the righteous judgment of God, that you may be considered worthy of the kingdom of God, for which you are also suffering—
2 Thessalonians 1:5
These words were meant to encourage a church that was faithful to the gospel despite suffering in extreme persecution. The early church saw its leaders and members imprisoned, tortured, and killed for the proclamation of the gospel that Jesus died and rose again for our sins. They suffered for the sake of the gospel story. Paul, who had endured numerous beatings and imprisonment himself sent them this reminder: the tide will turn one day.
Any time the church experiences persecution, it is because God has righteously deemed His people worthy of His kingdom so they suffer under man, while they remain faithful, loved and protected by God forever. Persecution means that Christians are doing something right. Jesus warned His disciples that the world would hate them just as it hated Him.
The tide will turn though. A righteous and holy God will call all people into account for their lives. Paul reminds us in this book that Jesus will return to judge this world. It is the theme of 2 Thessalonians. Jesus will call all those into account who do not obey the gospel (2 Thessalonians 1:8), and there will be dire consequences for rejecting Jesus (2 Thessalonians 1:9-10). We must passionately proclaim the gospel, faithfully, right up to that Day.