A doctrine is either of the world, the flesh and the devil, or of God (James 3:13-18 1 John 2:15-16, 4:1). This dichotomy is demonstrated by the following passages of Scripture:
Who is a wise man and endued with knowledge among you? let him shew out of a good conversation his works with meekness of wisdom. But if ye have bitter envying and strife in your hearts, glory not, and lie not against the truth. This wisdom descendeth not from above, but is earthly, sensual, devilish. For where envying and strife is, there is confusion and every evil work. But the wisdom that is from above is first pure, then peaceable, gentle, and easy to be intreated, full of mercy and good fruits, without partiality, and without hypocrisy. And the fruit of righteousness is sown in peace of them that make peace (James 3:13-18).
Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world (1 John 2:15-16).
Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world (1 John 4:1).
Thus, it is very very clear that any doctrine of the world is evil, and not to be accepted by any true follower of Christ, or even condoned, or tolerated by any follower of Christ. For to even tolerate, or condone such doctrines is itself to be an enemy of God. The Bible is inerrant which all in the true universal Church would accept. However, it is not enough to accept that it is inerrant. Anyone who wants to be a follower of Christ and serve God must, not should, but must, believe that it is sufficient, that is not only is true, but that there is absolutely no additional truth to the Bible, which can be sourced any where else. The only Truth is the Bible and all true spiritual revelation which truly comes from God.
Anything, even that which is not contradicting it, or even agreeing in part with some of the Bible, is not true. As such, such doctrines must be rejected by the follower of Christ. There is no such thing as "personal truth", a doctrine of post-modernism, which has infected much of the modern Church in the west, manifested in the belief that if one believes something, or feels something, it is true, that is true for oneself. This disease of post-modernism infects even reformed calvinist evangelicals who think that they are the most Biblical and theologically correct people in the whole of Christendom, but yet think that whenever they think something, for example, that whenever something offends them, it must be false. Such is the vile hypocrisy of such reformed calvinist evangelicals!
The doctrine of "separation of church and state" comes from the so-called 'enlightenment' era, which is an era which rejected the Word of God as backwards, oppressive, repressive, and having no place in society. To even think that the Word of God has no place in society is itself to hate God, and is the height of all arrogance, thinking that one need not God. The doctrine of "separation of church and state" purports the government is entitled to makes it own laws as it sees fit, without having regard to what the Word of God requires for all of humankind. The state, in other words, need not, or should seek counsel from the true Church of Jesus Christ in making laws, punishing evil and rewarding good. However, of course, in a somewhat ironic way, the state is entitled to stop the Church from doing things which offends it, or general society, such as speaking up against sodomite "marriage" or against the decriminalisation of bestiality, which is now starting to occur in some nations of the world.
However, what is more disturbing is that this doctrine is often credited to John Locke, a (professing) Christian, who satan deceived and used to spread his deception, through not the secular humanists, but through some professing Christian. That is just how subtly satan deceives. That a (professing) Christian first advocated for separation of church and state, if that is even true, does not in any way mean that the doctrine of "separation of church and state" is Biblical at all. Do not ever be deceived, just because a well-respected Christian leader advocates for a particular idea does not mean it is pure and Biblical. Do not even fall into leader worship, where one just loves a particular Christian leader so much that one falls into worshiping him.
The Bible tells us that government is ordained by God, not man. Romans 13:1-7 makes this clear:
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. Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist shall receive to themselves damnation.
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: 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.
Wherefore ye must needs be subject, not only for wrath, but also for conscience sake. For for this cause pay ye tribute also: for they are God's ministers, attending continually upon this very thing. Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour.
There is no argument that government is ordained by God as Scripture makes it unequivocal. Romans 13:1-7 is not simply referring to government during the time of writing in the first century. Neither it is referring to the Old Testament, for those of you who love to simply argue that all these verses that you find hard to accept are only applicable during the Old Testament, or during the time of writing for verses in the New Testament. No! Nowhere does the Bible ever indicate Romans 13:1-7 applies only during the first century.
This is because it is commanding the Christian to submit to authorities, which has absolutely nothing to do with culture or society. It has all to do with holiness and righteousness. Peter, whose writings are the same as that of Apostle Paul's (2 Peter 3:15), wrote in 1 Peter 2:13-16:
Submit yourselves to every ordinance of man for the Lord's sake: whether it be to the king, as supreme; Or unto governors, as unto them that are sent by him for the punishment of evildoers, and for the praise of them that do well. For so is the will of God, that with well doing ye may put to silence the ignorance of foolish men: As free, and not using your liberty for a cloke of maliciousness, but as the servants of God.
Governments are ordained by God to punish that which is evil in the eyes of God and to reward that which is good in the eyes of God (Romans 13:3-4; 1 Peter 2:14). They are ministers of God, placed by God to ensure righteousness in society. As such, the role of government is not only ordained by God, as per the Scripture, not by man. The role of government according to the Bible, has not, nor will ever change.
"Separation of church and state" is a doctrine of demons, which is a ruse used to intimidate the Church into submitting to the evils of secular humanism. This has lead to the manifestation of all kinds of evils in the modern world, for which governments now have to make laws against, owing to such evils being even conceived in the minds of many, as a result of the counsel of the Church being rejected by society.