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.
The "church" (Greek ekklesia) is a group of people, Jews and Gentiles equally, whom God has called out of the mass of humanity for a life separated unto Himself. The Greek word refers to many different types of assemblies (social, political, and religious), and in the Septuagint it is a synonym for "synagogue." This term became useful to Paul in gaining access to the Gentile world as well as in separating from the Jewish world.