Not everything about the kingdom of the world is bad. Insofar as versions of the kingdom of the world use their power of the sword to preserver and promote law, order, and justice, they are good. But the kingdom of the world, by definition, can never be the kingdom of God. It doesn’t matter that we judge it good because it stands for the principles we deem important — “liberty and justice for all,” for example. No version of the kingdom of the world, however comparatively good it may be, can protect its self-interests while loving its enemies, turning the other cheek, going the extra mile, or blessing those who persecute it. Yet [that] is precisely what kingdom-of-God citizens are called to do. It’s what it means to be Christian. By definition, therefore, you can no more have a Christian worldly government than you can have a Christian petunia or aardvark. A nation may have noble ideals and be committed to just principles, but it’s not for this reason Christian.

Gregory A. "Greg" Boyd (b. 1957) American evangelical pastor, Christian theologian, author.

The Myth of a Christian Nation (2007)