In 2 Thessalonians 1:1, it's interesting to note that unlike other letters of Paul, he adds nothing to his name. He doesn’t say, "Paul, called to be an apostle"; "Paul, an apostle...by the will of God"; "Paul, a servant of Jesus Christ." Those familiar things by which he designates himself are omitted here.
By this he's showing that his apostleship, his call, role, title, leadership and office were not in question among the Thessalonian church, so he didn't need to make any reference to it. But Paul's apostleship is constantly in question today, despite his words in Romans 11:13:
Paul is the apostle of the nations, the Gentiles. Paul, by the inspiration of the Holy Spirit, magnifies his office. We are to do the same. Paul was called by the will of God for this dispensation of grace. He was called to be an apostle for Christ to reveal to him the revelation of the mystery, the body of truth for this age, and for God to reveal His Son in him according to Christ's heavenly ministry today.
Paul is the one apostle of this dispensation. He is our apostle. Christ has revealed His will through Paul's thirteen letters for us to know what is Christ's mind, will, and heart for His Church, the Body of Christ, under grace. Is there confusion about Paul's role in your church? Perhaps your church knows Paul is our apostle, but makes no mention of it? If Paul wrote a letter to your church, how would he address it?