To such a one we must say: Certainly it is true that Christians, so far as they themselves are concerned, are subject neither to law nor sword, and have need of neither. But take heed and first fill the world with real Christians before you attempt to rule it in a Christian and evangelical manner. This you will never accomplish; for the world and the masses are and always will be unchristian, 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. Hence, a man who would venture to govern an entire country or the world with the gospel would be like a shepherd who should put together in one fold wolves, lions, eagles, and sheep, and let them mingle freely with one another, saying, “Help yourselves, and be good and peaceful toward one another. The fold is open, there is plenty of food. You need have no fear of dogs and clubs.” The sheep would doubtless keep the peace and allow themselves to be fed and governed peacefully, but they would not live long, nor would one beast survive another.
For this reason one must carefully distinguish between these two governments. Both must be permitted to remain; the one to produce righteousness, the other to bring about external peace and prevent evil deeds. Neither one is sufficient in the world without the other. No one can become righteous in the sight of God by means of the temporal government, without Christ's spiritual government. Christ's government does not extend over all men; rather, Christians are always a minority in the midst of non-Christians. Now where temporal government or law alone prevails, there sheer hypocrisy is inevitable, even though the commandments be God's very own. For without the Holy Spirit in the heart no one becomes truly righteous, no matter how fine the works he does. On the other hand, where the spiritual government alone prevails over land and people, there wickedness is given free rein and the door is open for all manner of rascality, for the world as a whole cannot receive or comprehend it. "
— Martin Luther (Luther and Calvin on Secular Authority)