What does 2 Thessalonians 1:1 mean?

As was customary in the first century, Paul identifies himself at the beginning of his letter. In several of his letters he calls himself an apostle of Jesus Christ. Here, Paul omits the term, likely because he did not have to defend his apostleship to this particular group. His readers were confident that God had called Paul to be an apostle. Silvanus (Silas) and Timothy had been with Paul when he preached in Thessalonica (Acts 17:4, 10, 14) and were with him when he wrote 1 Thessalonians. Now, they are with him as he writes 2 Thessalonians. Silas, Paul's missionary companion, had been imprisoned with Paul at Philippi (Acts 16:19–40). Timothy, also Paul's missionary companion, was from Lystra. His mother was Jewish, but his father was a Gentile. He joined Paul in missions after Paul circumcised him (Acts 16:1–3).

Paul addresses 2 Thessalonians to "the church of the Thessalonians in God the Father and the Lord Jesus Christ." The word "church" in the original language is ekklesia, meaning "called out ones" or "assembly." God had called the Thessalonian believers out of the world to be in Him and in His Son, the Lord Jesus Christ. In John 17:16, when Jesus prayed to the Father on behalf of His followers, He stated, "They are not of the world, just as I am not of the world."