Christianity grew up as an urban religion. It spread from city to city through the marvelous system of Roman roads, aided by a period of universal peace (Pax Romana) in an empire that spanned three continents. It was the rural areas that clung fiercely to the old religion of many gods. The name for these country people was ‘pagans’. The name stuck for all those who refused to follow the Christian religion.
Because Christianity grew up in a diverse society much like our own here in the US and in Western Europe, it learned to exist in a society as a minority. The New Testament letters urge Christians to respect the civil authorities unless those civil authorities ask of Christians something they cannot give: allegiance to another god but the God of Abraham, Isaac and Jacob.
Hence when St. Paul summarizes the Christian Credo as saying “Jesus is Lord” he was both asserting the Lordship of God over all human institutions and also warning civil authorities that this was a boundary point that Christians could not ignore. This simple phrase caused three centuries of persecution at the hand of the Roman Empire that required its citizens and non-citizens to make the assertion: “Caesar is Lord”.
But outside of that, Christians tried to fit into the society of that time without demanding that the Empire mirror all the virtues that Christians espoused. This is why Christianity can exist very nicely in a diverse, secular environment. It grew up in one and it learned that one is converted more deeply by persuasion than by edict. Of course, when Christians became the majority, they tried to make the civil world resemble the Christian ethos, even beginning to do to other faiths what the Romans did to them for three hundred years.
Theologically, much of this is based on the Lord’s response to the trap set by the Pharisees regarding the payment of taxes to Caesar: Give to Caesar what is Caesar’s and to God what is God’s. This, however, causes a great problem for other faiths especially for the other great monotheistic faith: Islam.
I would imagine (though I have not formally studied the matter) that the response of Islam to the ‘Render to Caesar what is Caesar’s and to God what is God’s’ is that there is nothing that properly belongs to Caesar. All belongs to God. Hence there cannot be any political government that is not under the authority of God. Separation of Church and State is impossible for much of Muslim Theology, hence the whole world of Sharia law.
Now the only way this vision works is when all the inhabitants of a state are of the same faith (with others in an inferior second level of political existence). When a theology such as this (Islam for instance) confronts the secular world and the United States particularly, there are problems that their theologians will have to work out.
Using the analogy of Islam: how can a Muslim accept this separation of Church and State?
Theologically there is a very important point in Muslim theology: everything belongs to God. But how do we live this in a developing and imperfect world where people are at all sorts of different levels of acceptance of this fact. Do we impose that vision of God’s sovereignty as ISIS or as other extremists would do?
Lest we think this mentality is only Muslim, we went through a time when it was considered ideal that everyone be Catholic or Protestant and we made laws based on those religious creeds that non-Catholics or non-Protestants would have to keep. There was even a time in the Church when the separation of Church and State was considered suspect or even anti-Catholic (think of Spain under the Inquisition).
Even in Israel, conceived as a secular state, there are political voices calling for it to become a ‘Jewish State’. What then becomes of the many Israeli Muslims and Christians? Would they have to be regarded as second-class citizens if citizens at all?
One wonders if any state or country in today’s world can ever be limited by ethnic or religious identity. Given the global movements of peoples and the ease of travel today, we are confronted with the need to provide structures of government that serve and do not impede this diversity. The Roman Empire, because of its excellent road system, also found itself in a diverse situation. Until the cult of Caesar arose, everyone was allowed to practice their own faith as long as it did not interfere with the administration of Roman justice or with the security of the Empire.
This was the environment in which Christianity grew. There are many similarities between that world and our own. Hence the separation of Church and State provides the Christian Faith with the room for growth that it needs. Secularism is the Empire without the gods and goddesses.
Christians must see that until the Second Coming of Christ at the end of time, every society is imperfect and every individual must be given room to come to a free decision to accept Christ. This is why the separation of Church and State is so important. It ensures that everyone has the free opportunity to choose the right and the good and the true.
But what if the secular world develops an ideology that is militantly atheist? Then it has become another religion and its errors must be exposed and confronted like the early Christians did for three centuries and as many religious people did in the Atheistic regimes of Communism and National Socialism: Christian martyrdom that claims no other innocent victims (like suicide terrorists do) but, based on a personal decision of faith, accepts one’s death rather than compromise and uses it to persuade others of the truth of that faith and the hopelessness of trying to force one to go against what they truly believe.