. . .for the world and the masses are and always will be un-Christian, even if they are all baptized and Christian in name. Christians are few and far between (as the saying is). Therefore it is out of the question that there should be a common Christian government over the whole world, or indeed over a single country or any considerable body of people, for the wicked always outnumber the good.Luther recognized that a "Christian" government required not just a statement of faith or a shared creed, but that the various citizens of that particular place had to also be committed followers of Jesus who lived not just under the laws of the state, but who held themselves to a much higher standard. If the majority of people, or even a significant minority, are not committed followers of Jesus, Luther says, you can't have a "Christian" government.
And Luther wisely distinguishes between those who are Christian "in name" and those who are authentically Christian. This is the key to understanding why an attempt to re-form the United States as a "Christian" nation is futile, mis-directed, and non-biblical. It flies in the face of the very nature of Christianity to be able to label a huge country as Christian. At most, you will enforce a weak agreement of some minimum standards of behavior and morality and basic creeds about the existence of God.
This is far from a life-giving relationship with Jesus in which individuals and small community groups live under the Lordship and authority of Jesus Christ by the power of his Spirit. Anything less than this is not fully Christian.