Suppose, uncontroversially, that it is morally obligatory to refrain from attempts to impose any particular religion on people by force. Call that obligation 'O'. What is the basis for O?
Some argue that the basis for O is a larger set of moral norms one can show, by philosophical reasoning alone, to be obligatory in itself and to logically entail O. But there is more than one version of such an argument. One is deontological and runs roughly like this: (i) Trying to force people to violate their conscience is intrinsically wrong, irrespective of what good might be thought to come of doing so, and thus can never be justified; (ii) trying to force people to adopt a particular religion is tantamount to trying to force them to violate their conscience; (iii) therefore, O. Let us grant that the argument is logically valid and that (ii) is true. Even so, there are two difficulties with (i).
One difficulty is that there's no agreement that (i) can be established on purely philosophical grounds without appealing to some sort of religious belief. If (i) itself requires religious support, then a regime upholding religious liberty of the sort described by O entails requiring religious toleration in practice from people who believe that such toleration is immoral—think Muslims and militant non-believers here—and also entails requiring such toleration for religious reasons. It's hard to see how that is compatible with O. The other difficulty is that, among those who believe O requires no religious support, there is no consensus about the philosophical basis for (i). Should religious liberty be made to rest on such a thin reed—especially given the all-too-likely possibility that some people have a culpably malformed conscience?
In view of such difficulties, some people prefer to support O in utilitarian terms. One way to do that is to make the common argument that seeking to impose any religion by force "doesn't work," inasmuch as such an attempt can, at most, achieve external conformity rather than genuine conviction. But whether or not that is true, the argument can be made to support O only if it is shown further that imposing external conformity fails to achieve social benefits outweighing the cost of doing so. Since that is precisely what the opponent of O would deny, the most that the "it doesn't work" argument can establish is that the proponents of O disagree with its opponents about what the relevant criteria of social utility are. Such an argument thus begs the question, unless some independent agreement can be reached about the relevant criteria.
By far the most common way of seeking such agreement is to argue that, given the very lack of agreement about what the objective basis for O is, society is better off treating O as a working assumption needed for the sake of maintaining civil peace. And given the long peace about religion that has prevailed in the West, that argument seems plausible. But there's a problem nonetheless. It is not only conceivable, but in fact sometimes happens, that civil peace cannot be maintained by premising a regime of religious liberty on O.
Have a look at what's going on in countries with significant Muslim populations agitating for the imposition of sharia, at least for themselves. Confronted with such agitation, a regime premised on O for the sake of maintaining civil peace can no longer cite utilitarian reasons for doing so. It must either accede to Muslim demands for the sake of maintaining civil peace, thus imposing a religious double standard that is incompatible with O itself; or it must refuse such demands, in which case civil peace is concretely undermined because the Muslims believe themselves to be the victim of religious intolerance. Accordingly and paradoxically, a utilitarian rationale for adopting O only works if nearly everybody in a given society believes O for their own reasons anyway. When such a consensus breaks down, so does the utilitarian rationale for O.
Much the same would go for a society in which it is not the Islamists, but rather militant secularists, who hold sway. As the example of Canada illustrates, a regime theoretically premised on O, but practically controlled by secularists, can easily end up curtailing the public profession of forms of Christianity that secularists and other non-Christians find offensive. In addition to being arguably incompatible with O, such an outcome is consistent with civil peace only so long as such Christians remain sheeplike. But there is no practical guarantee that they will remain sheeplike, and no moral guarantee that they should.
In light of all this, it seems clear that the dual challenge of Islamism and militant secularism requires Westerners in general, and Americans in particular, to think more rigorously about what we believe the objective basis for religious liberty to be. So far, I don't see much of that happening.