Back in Acts 17, we read of Paul, Silas and Timothy founding the church in Thessalonica. Several of Paul’s letters, also called epistles, are written to churches Paul founded to address issues that have come up since Paul left those cities. From Acts 17, we see that Paul left Thessalonica abruptly. He probably was concerned he had not spent enough time there to get them well established. According to 1 Thessalonians 3:6, he has received a good report about the church and is telling them how happy he is to hear they are doing well.
Faith in Jesus is something that is shared with people all over the world. We can pray for other Christians, no matter where they live and rejoice when we hear they are doing well. Wherever the church exists, it should be representing Jesus.