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 Greeks used the term "church" to refer to any "called-out assembly," whether political or civil. Paul gives the term a specialized meaning. The addressees are clearly not a pagan assembly, although many of them have previously been pagans (verse 9), for they are "in God the Father." Nor are they Jewish because they are "in the Lord Jesus Christ," although some of the first converts in Thessalonica came from the synagogue (Acts 17:4).