Religion and nation generally deal with different aspects of life. Religion or rather, the scriptural basis which is generally termed as 'religion' deals with the matters that are spiritual, and all that runs in the mind and memory. The nation, or to be specific, the Constitution of the country deals with various aspects of the law and rules and procedures, etc., the consequences for abusing the given rules, and so on.
Both can be used together in the governance of countries, and it can be very productive if the religious aspects are removed, and the scriptural principles are put to use. Religion and religious policies get changed to suit the personal benefit of a few. Scriptures do not allow that.
In the Bible, scriptures played a major role in the olden times, and governance was based upon the doctrines described in the book. Since Jesus' time, there is a huge difference in how believers are to operate in the countries they are in.
Jesus, at the questioning by the Roman ruler before his crucifixion, states that "….My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews…."John 18:36. This is a very important point to remember. It means that the statements some say, (usually from western countries and some movies) that this country needs to be won for Christianity. That goes directly against what Jesus says. Jesus asks his followers to teach the good news about the savior and the free salvation that is available for all. He never spoke about becoming rulers upon earth. A section of people at his time wanted to make him King but he would not allow that. Nor did he ask his followers to build up religious buildings and all that goes with their maintenance. He never taught anywhere to despise people's beliefs. But he was often teaching people and followers to fear not, to believe in God's kindness and went about doing good. He also spoke harshly against the religious rulers of his days, who were most interested in preserving their power, wealth and religious control over people. Matthew chapter 23 lists them out.
The New Testament also teaches believers to: Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God (Romans 13:1). This means in our country, to pray for the rulers to rule well and care for people. (Even if a person may vote as per his choice)
3 For rulers are not a terror to good works, but to the evil. Wilt thou then not be afraid of the power? do that which is good, and thou shalt have praise of the same:
4 For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil.
7 Render therefore to all their dues: tribute (= taxes) to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour.
Jesus calls his followers the 'salt of the earth' whose purpose is obvious. His kingdom is within you, in hearts and mind; not a land area.
Here are some examples of 'religion' spoiling everything. Within 300 years after Jesus, the Christianity became a popular thing and became 'official', and so if anyone wanted a Roman government job, he needed to be a Christian. Soon enough the whole system got corrupted by religion and tradition to lead into the Dark Ages. That went on till somebody read the scriptures to pull everybody out of the religious mess to true following of Jesus. Again that went down till another man pulled out of the newly established religion rule and so on it went.
The UK and USA were based on Bible principles originally, and then became a religious empire/power, and most of us know what such religious nations did and do everywhere. In fact in those countries you cannot have Bible in schools, but you allowed having Satan worship and other evil rituals freely.
About 2 years ago there was a debate on TV with that shouting man as host, where one Bible believing govt officer was asked if the Bible was greater than the Constitution. The above scriptures answer for that. One cannot really put the Constitution and Bible on same level – they deal primarily with different aspects and with some minor overlap.