I recently picked up Paul Weithman's book, 'Religion and the Obligations of Citizenship'. This is a topic which has been very interesting to me in the past. My understanding is that Weithman is very studied in Rawls and comes at politics from a left-leaning Christian perspective. I've wanted to learn more about this perspective lately, especially as I find myself more open than ever to learning from this point of view. Here's a quote I liked at the very beginning:
"I began this book hoping to make room in the theory of liberal democratic citizenship for saints and heroes of the religious left, such as Dorothy Day and Martin Luther King. I was troubled by theories which seemed to imply that such people violate their civic duties by engaging in religiously motivated activism or by putting forward exclusively religious arguments. I was also troubled by the thought that theories which do seem to accommodate them do not do so in the right way. Much to my surprise, I felt driven to different answers about religion's role in democratic politics than those I had previously accepted and to a much less moralized view of citizens' proper relations to one another."
So on a Rawlsian picture of political debate, it is somehow illegitimate of citizens to appeal to religious reasons not shared by the rest of one's political community (at least, that is my understanding of how it works in 'A Theory of Justice'). In other words, if one bases one's political views solely or primarily on religious grounds, then one is to some degree being a bad citizen.
But this seems implausible. For one thing, it seems unable to accommodate many heroes of the left who were deeply religious. For instance, MLK is a particularly good example of how a citizen should engage in public discourse, yet the bulk of his arguments and rhetoric were extremely religious.
It seems instead that what really matters then is whether the views which are concluded to on the basis of the religious arguments are themselves good ones. You might respond that what is important is that the religiously motivated political views have good independent arguments (though I worry about this proposal). However, even in that case, the mere fact that some people hold those views based primarily on religious arguments will not be enough to discount those citizens' holding them. And after all, some religions might be true; in that case, it seems their canons are certainly relevant to political discussion! (If you say none of them are, then it might make sense to say they should be discounted in political discourse; but this would seem to itself be a "religiously motivated" political view.)
If I recall from my reading long ago, Michael Sandel makes a very closely related argument in his own book, 'Liberalism and the Limits of Justice'. The point about heroes of the religious left seems to me to be a very serious problem as regards certain liberal political theories on the proper relation between religion and politics; a decisive counter-example, if you will.
Of course, probably one of the biggest motivations for espousing the Rawlsian type of view here is to rule out things like religious fundamentalism (in the American context, specifically Christian fundamentalism seems to be a relevant worry). However, that can be much better accomplished by simply pointing out that fundamentalism is bad ethics, bad science, and frankly bad religion. There is no need to argue against certain specific politico-religious views by arguing against politico-religious views in general; indeed, that seems to be a big mistake, as the MLK case shows. One simply must engage with religious views on the basis of whether they and their entailments are right or not.
With all of that said, I'm very interested in seeing what a political philosophy of the religious left might look like; one which doesn't incorporate, say, Rawlsian ideas about religion and public reason into the equation. I'm very excited to read further on in Weithman's book.