Such a government appears to be in contradiction to Christ's own words, where He said "My kingdom is not of this world." The issue isn't that the state is a master in competition with Christ, it is a government appointed by God just as much as Domitian or Nero were. Where the serving of two masters comes in is committing oneself to the national interests of a state under nationalism since God has not set up a vassal kingdom in the New Covenant. There exists no Christian state, and such a thing would require God instituting it not it being thrust upon the land by human beings. So commiting oneself to the interests of a nation means excluding and going against other parts of the body of Christ.
The problem is that the early church didn't volunteer their families into the political-theological education of the Roman state. Government was something to be obeyed and to pay taxes, but you weren't supposed to surrender your hearts and minds over to them.
But Christians have been doing that with the modern liberal-democratic state for a long time. Christians celebrate the violent revolution of America's founding as a great ushering in of Liberty in the world. Christians volunteer their children over to the social-engineering of the secular state, for example, children learn that women have been 'liberated' through modern social reforms and that any suggestion that men should be the leaders in society is evil/authoritarian/fascist, etc. so Christians have accepted the moral framework of liberal democracy at the expense of a Christian framework.
You refuse to have a Christian state, but you also refuse to have a Christian community that is culturally set apart from the secular state.
The secular state is a "moralistic therapeutic state" and its managerial class informs and disciplines the Christian community on how to adjust and adapt their moral framework to that of the values of liberal democracy.
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