Rawls thought everyone could live happily, just so long as the religious always justified their political ideas in secular terms. As Atrios put it, "[A]s long as it stays away from policy I really don't care what people believe or choose to worship."
The trouble is that no believer is going to accept this proviso. In effect, Rawls is either asking the religious not to be motivated by their most fundamental convictions, or to put up justifications that are disingenuous. If that's what's necessary to allow for deep religious pluralism, then it doesn't seem to have much of a shot.
Jurgen Habermas recognizes this criticism of Rawls. According to the German, for religion to co-exist with secularism requires something of a transformation of both religion and secularism.
The first transformation is to religion. As it ceases to be the unquestioned background belief of everyone, and is challenged by science and liberalism, believers are forced to undergo a critical, reflective attitude towards their own traditions. That is, in effect, what theology is, although Habermas further requires that theology somehow diffuse down to the ordinary believer.
However, Habermas argues that the secular must undergo a similar process. This is not merely a question of being respectful to other people's convictions. Rather, it is a realization of how contingent the secular liberal's own ground is:
"As long as secular citizens are convinced that religious traditions and religious communities are to a certain extent archaic relics of pre-modern societies that continue to exist in the present, they will understand freedom of religion as the cultural version of the conservation of a species in danger of becoming extinct. From their viewpoint, religion no longer has any intrinsic justification to exist. And the principle of the separation of state and church can for them only have the lacist meaning of sparing indifference. In the secularist reading, we can envisage that, in the long run, religious views will inevitably melt under the sun of scientific criticism and that religious communities will not be able to withstand the pressures of some unstoppable cultural and social modernization. Citizens who adopt such an epistemic stance toward religion can obviously no longer be expected to take religious contributions to contentious political issues seriously ..."
But this Dawkinsian state of secularist innocence cannot survive confrontation with the stubbornness of religious belief.
[T]he insight by secular citizens that they live in a post-secular society that is epistemically adjusted to the continued existence of religious communities first requires a change in mentality that is no less cognitively exacting than the adaptation of religious awareness to the challenges of an ever more secularized environment.
Both these exacting cognitive processes can be refused. The believer can retreat into fundamentalism. We know about that.
The secularist who refuses the exacting cognitive process Habermas demands? What does he or she do?
Well, one response is extreme Islamophobia, the "liberalism of fools". Of course, while there, common cause ends up being made with fundamentalist of the other Abrahamic religions. It all gets messy, as Christopher Hitchens will no doubt inform us all when he finally sobers up.
Another response is that of the Eschaton commenter. There are many good reasons to dislike the Bush administration, but part of the rage -- it must be admitted -- comes from the fact that Bush's electoral victories make it impossible to imagine America trending blissfully towards European levels of secularism. Not that the Europeans are really that secular, as opposed to post-Christian.
Habermas concludes with the thought that the co-existence of a religious and a secular tradition is not a matter of normative argument, but of the actual history of particular religious and particular secular traditions. There is no great optimism there that all religious traditions (or even any of them) will find a workable accommodation, or that all secular traditions are capable of responding to the religions that will try.
Update: This Brad De Long thread shows that even highly intelligent and thoughtful liberals can be absurd when it comes to religion. De Long links to a dumb post by P.Z. Meyers in response to Terry Eagleton's evisceration of Richard Dawkins, in which Meyers claims that theologians are dishonest. A fellow by the name of Kent argues agains De Long, earning himself a disenvowellment, the label "troll" and the question "What makes you think you know something about theology?" Turns out "Kent" has a Ph.D. in the subject. Oops.
I'd note that this is from one of the most thoughtful and academic voices in the left blogosphere.