The greetings are addressed “to the church of (the) Thessalonians in God the Father and the Lord Jesus Christ.” No such address had ever been written or read before, for the community to which it was directed was a new thing in the world. The word translated “church ” was certainly familiar enough to all who knew Greek: it was the name given to the citizens of a Greek town assembled for public business; it is the name given in the Greek Bible either to the children of Israel as the congregation of Jehovah, or to any gathering of them for a special purpose; but here it obtains a new significance. The church of the Thessalonians is a church in God the Father and the Lord Jesus Christ. It is the common relation of its members to God the Father and the Lord Jesus Christ which constitutes them a church in the sense of the Apostle: in contradistinction from all other associations or societies, they form a Christian community. The Jews who met from Sabbath to Sabbath in the synagogue were a church; they were one in the acknowledgment of the Living God, and in their observance of His law; God, as revealed in the Old Testament and in the polity of Israel, was the element or atmosphere of their spiritual life. The citizens of Thessalonica, who met in the theatre to discuss their political interests, were a “church”; they were one in recognizing the same constitution and the same ends of civic life; it was in that constitution, in the pursuit of those ends, that they found the atmosphere in which they lived. Paul in this Epistle greets a community distinct from either of these. It is not civic, but religious; though religious, it is neither pagan nor Jewish; it is an original creation, new in its bond of union, in the law by which it lives, in the objects at which it aims; a church in God the Father and in the Lord Jesus Christ.
 James Denny, The Epistles to the Thessalonians (1899), 8-9.
The lecture notes may be found here:Lesson 5 The Church is a Creation-3