"It should be no surprise that the Romans treated the church as a political threat whose practices were subversive of good order in the Empire. Pliny, in a letter to the Emperor Trajan (c. 110 CE), reports that he applied Trajan's ban on political societies to the Christian communities of Asia Minor. In the Roman view, Christian failure to worship the pagan gods and their assumption that allegiance to Caesar conflicted with allegiance to Christ was not simply a religious matter, but concerned imperial political order. As N.T. Wright notes, Christians did not attempt to defend themselves from persecution with the claim that they were merely a 'private club' or collegium for the advancement of particular interests. They continued to proclaim the kingship of Christ, even if such kingship was not based on the model of Caesar's (Wright 1992: 346-57). That Christ's kingdom is not of (ek) the world (John 18:36) was regarded as a statement of origin; the kingdom is not from the world, but it is in the world and deeply concerned with it."
William T. Cavanaugh, "The Church," in The Blackwell Companion to Political Theology (Oxford: Blackwell, 2004), 396.