The Enlightenment idea of a separation of church and state is currently under attack from several directions. Theocrats, communitarians, and anti-foundationalist radical intellectuals all claim that the idea lacks foundations, and for different reasons they all seek to sweep it away as a plank in the structure of the modern liberal state. Some of these people, of course, are highly illiberal in their worldviews. I'll get back to that in my final comments.
J. Judd Owen's book Religion and the Demise of Liberal Rationalism: The Foundational Crisis of Church and State includes a powerful response to anti-foundationalist critiques of the vaunted church/state separation, placing it in its orginal seventeenth and eighteenth century context. Whether the idea needs to be abandoned, modified, or strengthened, in current social and political circumstances, it will help us if we can clarify what it was originally all about.
Owen's account chimes well with my understanding of the Enlightenment and my own liberal values, so I'm simply going to synthesise and summarise it here for future reference ... before offering a few paras of comments in response.
He observes that the principal political ambition of the liberal Enlightenment was to create a reasonable political order by ending the worst consequences of the fanatical, clashing religious orthodoxies: consequences such as persecution and bloodshed. For John Locke, writing in support of religious toleration, it was a fact that no universal agreement could be forthcoming on the ultimate question of salvation of the soul. From this, he attempted to bracket off intractable religious disputes from practical politics.
For Locke, temporal authority trumped spiritual authority, and the limit of toleration could be determined by the need to conserve the political order. He found the basis for political common ground in a distinction between the goods of this world and those of the next. The goods of this world, such as the security of earthly possessions, are the sole business of the state, while the salvation of souls is the sole business of the church (in its various sects). Failure to make such a distinction, and impose a religious orthodoxy, leads to volatile political conflicts here on earth.
This raises the question as to why the might of the state should not be directed to both ends, temporal and spiritual. Locke's answer considered a difference in the ways these ends could be secured - one would not trust one's soul to whatever magistrate was in power, so long as the limits of understanding rendered theological disputes intractable. By contrast, the state had means that were eminently capable of dealing with such problems as criminal violence, theft, and fraud, or the incursions of external enemies.
However, what about the spectre of religious zealots who would not be content to restrict the state to such secular ends as peace and security, and would simply deny that these were the highest political ends? Locke's response was that it was not religion as such that caused political disturbances, but oppression and persecution - if the threat of these could be removed, the various sects would become peaceable. On this account, if people's primary needs - such things as safety and security - could be met, then human beings would be religious in varying degrees but not with revolutionary zeal.
The aim of the Enlightenment thinkers was to transform religion so as to remove its tendencies to violent zealotry. The state could do so not by initiating yet one more religious sect to be placed above the others, but by attempting to turn political attention away from theological quarrels and sectarianism, depoliticising the sects.
While Enlightenment liberalism sought to appear neutral on matters of religion - by not siding with a particular theological position - it did so in the service of a political end that was not value neutral.
Within the Enlightenment framework, the response of the liberal state to religious disruption is to deny all sects and churches the status of orthodoxy - which removes the ground for either persecutions or for the political hegemony of any sect in particular. The liberal state proceeds, strategically, to be neutral on religious questions. In a sense, religion is seen as too important to be left to whatever ruler or electoral majority prevails at a particular time.
As Judd emphasises, this is really a strategy, or a policy, rather than a theory. That is, there is no theoretical stance above and beyond whatever account of human nature is built into an analysis such as Locke's, plus the need to protect the solid, politically necessary value of civil peace, plus the practical effectiveness of the chosen course of action.
Such thinkers as Thomas Jefferson, who argued for a wall of separation, did not believe or intend that this strategy would have a neutral effect on the theological views of citizens. It was intended to soften the differences between contending sects, produce some blandness, and foster a degree of private indifference to religious orthodoxy. However, it also supported the right of individuals to seek out truth themselves, in discussion with the contending parties.
Now, some comments of my own. This general strategy has, indeed, served Western nations rather well since the eighteenth century, even though it has not been instituted everywhere as a formal separation of church and state, and even though it may not have been followed with total rigour in any jurisdiction. As carried out in practice, the Enlightenment approach may not be entirely satisfactory to everybody, or to anybody, but to the extent that it has grown from a practical strategy to a political principle (albeit one with no theoretically deep foundations) it seems worth defending. Most people can find a basis to give it at least some support from within their respective worldviews ... although, alas, there is no proof that every worldview can provide such reasons.
Even if we can't insist that people totally abandon their religious commitments in the political realm, we can at least ask them for a high degree of political tolerance for ideas that do not mesh well with their individual religious beliefs.
At the same time, ideas of church/state separation and religious tolerance do not provide a substitute, from anyone's viewpoint, for debate about the truth of particular religious and similar ideas. They certainly do not require that private actors must discuss the religious views of others with any special respect. Tolerance does not go that far.
Because the Enlightenment strategy is ultimately "just" a strategy, it is open to modification. We might even abandon it, if new circumstances so dictate. However, I suggest that we do not do so except for the most compelling reasons. The advice of Locke and Jefferson, and other Enlightenment figures, has worked too well, and the alternatives are too untried. While it may, technically, be a mistake to claim, as I sometimes do, that the relevant Enlightenment ideas are fundamental - we can tell a story about how they are historically justified - they are nonetheless enormously valuable and important.
I often read or hear secular thinkers who suggest various actions that the religious would surely experience as persecution, and I try to emphasis that religious tolerance includes secular people tolerating religious practices that we may not like, and to insist that separation of church and state goes both ways. I.e., the churches should not attempt to impose their contentious views using the coercive power of the state, but nor should state power be used to prevent people practising their religions.
As Locke knew, there are grey areas here: the state may have good secular reasons to take actions that will have a negative effect on some religious groups, even if its leadership is not motivated to carry out persecution for its own sake. As the role of the state has expanded beyond internal and external security for its citizens, the scope for clashes of public secular values and private religious ones has also increased. But there is room for sensitivity whenever a new issue arises. Sometimes, issues are best addressed by means other than the exercise of state power even if it appears legitimate.
Not to put to fine a point on it, my real concern is how all this stands up within the current world order, one very different from the Enlightenment era, though with some of the same problems as bedevilled pre-Enlightenment Europe. Today, liberal societies face several varieties of emboldened politicised theology, whether from American fundamentalist Christians, from the warriors of radical Islam, or from a conservative Vatican leadership with strong ambitions to influence the policies of Western governments. There can be few more important questions than the question of what strategies will best address this challenge and carry us through a difficult new century.