Separation of church and state is absolutely necessary to the continued health of a democracy or republic1. History has shown time and again that any time a religion is put in a position where it has political power, it will proceed to use that power in ways malevolent to opposing religions and ideologies, as well as to anything else it considers immoral or in some way a threat to its doctrines or authority.2

John Calvin preached religious liberty - right up until he gained power in Geneva, turning it into a theocratic dictatorship where only his personal brand of Christianity could be preached - what we today call Presbyterianism. In Russia, the Orthodox Church for centuries maintained a stranglehold on art and literature. The entire history of the Roman Catholic Church at least up to the Reformation is nothing less than a record of repression, oppression, atrocity and conquest in the name of the Messiah. And (since I seem to be concentrating exclusively on Christianity) let's not forget the brutal oppression in many Muslim nations - most notably Afghanistan under the recently-ousted Taliban, Iran under the ayatollahs, and Saudi Arabia - of women, of religious minorities (including Muslim sects which are a minority in a given country; or even a Muslim sect which is a majority but has no political power), of most anyone with differing cultural or moral values. Why? Not because Islam is necessarily less tolerant than Christianity or other religions (ever actually read what the Bible has to say about "nonbelievers"?), but because in most Muslim nations religious leaders are closely tied to the state (or in some cases, directly in command of it).

Moreover, collusion between the state and a religion will often cause the reduction or evaporation of religious liberty. Why has such a variety of religions and sects thrived in the United States? Because the state cannot dictate doctrine to a church, and because no church has the power to impose its doctrines on the masses by way of the state. This was, in fact, the primary reason that Christian members of the Continental Congress (who did after all greatly outnumber the Deists) agreed to the establishment clause in the First Amendment; nearly every colony wanted its own state church to be made the national church of the new nation, but given the choice between having no national church and having a rival church attain that status, the representatives preferred the former arrangement. Why? Because they knew, as I have written above, that when a religious entity is given political power, it automatically uses that power to destroy its opposition.

To put it succinctly: If you want to live in a theocracy, move to Iran. If you prefer to live in a place where all religious (and irreligious, for that matter) views are tolerated, you must live in a nation where this is a separation of church and state.

1: Someone is bound to point out that many democratic nations in Europe exist which have a state church. My response to this is; yes, there is a legal connection between the church and the state, but in most places it is archaic and state-church separation exists de facto for the most part. (The only notable exception I can think of is the continued teaching of Anglicanism in British public schools.) To make a comparison with another fact of modern government: In Britain, Queen Elizabeth II legally has control over all political power. That is to say, the Crown is considered the fount of the government's legitimacy. Legally, she could dissolve Parliament and declare herself absolute monarch of Britain. However in all practicality, if she did this, she would (I would hope) have little-to-no support from the people. Similarly, the law still on the books in Arizona that says two people of opposite sex can't room together unless they're related or married, while still having legality, is in reality archaic and unenforced and thus de facto has no legitimate power to keep two such people from doing that.

2: I am in no way suggesting that religious entities are the only such that tend to behave in this manner. For instance, under Stalinism, all religions and "subversive" (i.e. anti- or non-Stalinist) ideologies were banned in the Soviet Union (although you could argue, and many have, that communism itself is a kind of religion). I am only stating that most if not all religious entities behave in this way.

This part of the writeup is a response to dghallau's writeup above:

1) I am curious to see what evidence you have that there have been any siginificant trends towards atheism in the United States. I would find it especially interesting, considering that the ARIS 2001 poll1, which took a random sample of 50,000 people (allowing for a very, very tiny percentage of statistical error), found that approximately 2,000,000 Americans - or less than 1% - consider themselves atheists, agnostics, or otherwise nontheistic - and this is probably the most liberal number given in any such poll. Of course, if you include those who consider themselves "non-religious" - not a legitimate assumption to make when many people may be theists but not practitioners of an organized religion, although groups like American Atheists insist on doing it anyway - the number is 29.5 million, or about 14%; but even so, I don't believe this number has changed significantly in the last 30-40 years. Of course, I could be wrong.
2)In what way does government exist to promote morality? Do you believe the government should prosecute people for blasphemy or obscenity? Adultery? Recreational use of marijuana?

It seems to me the only extent to which the government has the legitimate right to promote a moral code is in reference to those actions which cause definite physical (as opposed to spiritual, not neglecting emotional and psychological) harm: murder, rape, child abuse, etc. Anything else is an infringement on the individual's rights and their moral autonomy.

For that matter, in what way is religion's aim to promote morality? Certainly, most religions prescribe a moral code in some shape or form, but that is not their primary purpose. The primary aim of a religion is to instill certain metaphysical and epistemological beliefs; the belief in a god, the belief in an afterlife or in reincarnation, the belief that the universe is not truly as it appears to our senses (idealism), etc. Moral standards are set within this framework: If your eschatology is that of a dichotomous heaven and hell, then there must be certain things you can do to get into heaven (or be trapped in hell).

More to the point, whose morality is it anyway? Even if I were to grant that the state and religions have as their ultimate aim the promotion of morality (and I don't), we still have to ask which morality is "best". Certainly the moral code of your average Christian fundamentalist is much different than that set laid out in the Humanist Manifestos. Who decides which is best when there are no objective standards of morality?

Finally: As I've said, all religions have their own metaphysical and epistemological doctrines which their followers necessarily believe in - and even if you disagree that this is the primary purpose of religion, I doubt you'll seriously contest that it is at least *one* of its main purposes. Under a theocracy - even the "truest and best", whatever that means - the religion in power can and almost certainly will enforce these views on the general public, possibly banning all opposing views. But what right does the government have to tell me whether or not I believe in God, or if I do what kind of God to believe in? What authority does Congress or the President have to determine whether I should believe in a heaven and hell, or in reincarnation, or in Valhalla, or in no afterlife at all? The answer is none; none whatsoever. But under a theocratic government, they would nonetheless have the power to do so. *That* is why seperation of church and state would always be necessary; even with the best of governments, even with the best of religions.

1: Source: