Every kingdom has its borders and border markings within which its subjects reside and by which they are separated from others. The kingdom of heaven is likewise separated from all nations, and functions independently without intertwining itself with other spheres of sovereignty. This separation does not pertain to locality, region, or city, as was true in the Old Testament, but she is separated from others by fellowship and confession. Thus, the church does not wish to have those within its community who still belong to the world or embrace a false religion. The church wishes to be separate in order that the kingdom of Christ may be all the more evident. . . .

As the church is separated from the rest of the world, she is likewise united internally, which is evident from the word congregation itself, it being expressive of her internal fellowship. This is evident by virtue of her separation, and by way of comparison she is called a house which is built by the uniting of individual stones. She is called a body, consisting of many united members; a flock, consisting not of sheep who are dispersed but who are gathered together; and a kingdom in which the subjects are united for mutual protection.

WILHELMUS À BRAKEL, THE CHRISTIAN’S REASONABLE SERVICE, VOL. 2, 21.