Paul, Silvanus, and Timothy, to the assembly of the Thessalonians in God the Father and the Lord Jesus Christ: Grace to you and peace from God our Father and the Lord Jesus Christ.
Paul. He does not call himself “an apostle,”... because his apostleship had never been called in question by the Thessalonians. For the same reason he omits this title in the Epistle to the Philippians; whereas he strongly insists upon it in his Epistles to the Corinthians and Galatians, because among them there were many opposed to his authority. PPC