One condition of peaceful residence in almost any nation is a willingness to respect and submit to the authority of the human magistrate and government. If this magistrate makes a law, the people are expected to obey it, or face the legal consequences and punishments. So it is strange indeed that so many today have a controversy with the requirement to submit to the divine magistrate, the God of the Bible, who has a claim over every nation and people. If people are willing to submit to mere men- who are far more capricious and far less just than Christ- why do they think it so strange that they must as residents of a country submit to the Sovereign of the Universe?
The moral standard by which Christ rules nations (just as it is the moral standard by which He rules churches) through civil magistrates is His Ten Commandments. The civil magistrate must enforce these Ten Commandments, for he has been charged with the duty and power to punish evil doers (Romans 13:4). He is a servant of God for the task of punishing evil, which is defined as a violation of the moral law summarized in the Ten Commandments (Romans 13:9). The first of these Commandments teach that man should have no other god than the God of the Bible (which is Christ). It is the duty of the civil magistrate to see that at least outwardly every citizen in his realm respects the authority of Christ. Not only this, each has a duty not to set up heathen idols, desecrate the Sabbath, commit murder, etc. Violations of any of these standards should be discouraged or suppressed by the civil magistrate.
This power of the civil magistrate does not necessitate that the State rules the Church or the Church rules the State, any more than it would be correct to say that the current executive branch of federal government rules over the judicial branch or the judicial branch rules over the executive branch. Each branch is separate and distinct, with its own given powers. The judicial branch explains and interprets the law, whereas the executive branch executes the law. The judicial branch hears cases; the executive branch supervises the armed forces. So the Church has powers such as preaching and explaining the Bible, administration of the sacraments, and excommunication; and the State has powers such executing corporal and capital punishment for crimes and supervising the police and military. Neither should assume the powers of the other, but ideally each should work together to promote righteousness as it is defined in the Ten Commandments.
An illustration will perhaps help show the rationale for Christian nations. Suppose a resident of a nation decided that he did not want to throw away his household garbage in an orderly manner. He did not want to place it in the trash can to be picked up by the waste management services. Instead, he decided to throw this garbage in the middle of the street. If the civil magistrate did not suppress such behavior then others as well would quite likely be emboldened to dump their garbage in the middle of the street. Soon such garbage would pose a serious health and safety risk for the community. So it is with moral garbage that is not discouraged and suppressed. If a citizen is allowed to desecrate the Sabbath, then over time others will be emboldened to do the same. Over time huge crowds will desecrate the Sabbath by engaging in spectator sports on Sunday. And what is true of the Sabbath is true of the other Commandments.
The myth which has caused so many to reject the idea of Christian nations is the notion that there can somehow be a neutral nation and civil magistrate. But this notion is a myth indeed. Every nation must be governed according to certain principles and standards. If those principles are not the Ten Commandments, then they will be the principles of some other world view- whether Islam, Judaism, Hinduism, secular humanism, deism, etc. There simply is no neutrality. Laws must be enforced (lest there be anarchy), and those laws must be derived from some over-arching world view.
One objection to Christian nations is the argument that to impose Christianity upon people is cruel and inhumane. This objection must be rejected because God himself will cause every knee to bow to Christ. Unless we are prepared to charge God with sin, then we must realize it is not cruel. Indeed, it is not only not cruel, it is merciful to impose Christ's law, for it is a law of liberty (James 2:12).
Another objection to Christian nations is the argument that the Covenant of Grace is not with all people in a given territory. It is indeed the case that in its spiritual essence God's covenant of grace is only with His elect. But in terms of its visible administration it comprises all who outwardly name the name of Christ and are outwardly under the authority of Christ's Ten Commandments.
Blessed indeed is the nation whose God is the Lord!
(More Puritan tracts like this one can be found at http://www.puritans.net/tracts.)