The standard liberal objection to religious motivations for political action is that they are unquestionable and not susceptible of disproof, so that they cannot form a part of the ongoing rational dialogue that should ideally characterize the political process. Indeed, the “special relationship” that secular liberalism posits between religion and violence is based precisely on the fact that religiously-motivated actions are not motivated by reason and hence are arbitrary and unpredictable — i.e., violent.
In one of Zizek’s weakest books, On Belief, he claims that liberals are actually the “believers” in this sense. He doesn’t back up this claim very effectively, choosing instead to indulge in misleading, “provocative” violations of liberal pieties, yet I think we can see that the core insight is there when we notice that the signature gesture of our ruling classes is to present themselves as the mere vessel of impersonal, ineluctable forces. Powerful, impossibly wealthy businessmen have no freedom of choice, as the market determines everything they do. Politicians are similarly guided by what is “politically possible,” irrespective of the range of options their office should theoretically give them.
Obviously it is human to try to beg off responsibility by pointing to forces beyond one’s control — but surely never before in history has a ruling class so thoroughly legitimated itself as constrained by forces beyond its control. It’s as though the one qualification for political or economic power is the ability to divine the messages coming from these powerful occult forces that guide our lives. Any actual deliberation about what should happen is radically foreclosed by this stance: indeed, proposing to debate openly about the shape of our shared life is painted with the same brush of fanaticism as in the liberal critique of religion, except this time the label is “populism” (a catch-all term that completely ignores the unmistakable differences between right- and left-wing principles and priorities).
I would venture to say that back when societies were structured according to religious principles and everyone basically believed in God, a political or business leader who claimed to be a direct channel for God’s will would’ve been regarded as either insane or dangerously disingenuous. Re-label “God’s will” as “the market” or “the politically feasible,” however, and no one bats an eye.
I’d further claim that in settings where religious authority factored significantly in the political process, debate was actually much more vigorous — just compare the Talmud to the editorial pages in a mainstream newspaper, for example. That’s because everyone recognized that the sources of religious authority, as was fitting for something from a divine source, were difficult for us mere humans to understand, so that our conclusions about God’s intent were almost always subject to error and reinterpretation.
Not so with the contemporary impersonal deity who inspires our ruling elites! It’s always right there in the numbers, in black and white. There’s no room for interpretation or debate, unless that means using more sophisticated (and hence reliable!) mathematical tools — at the end of the day, all you need is a literal interpretation and you’re good to go. No religious fundamentalist can possibly be as closed off to alternatives as the secular liberal fundamentalist armed with absolute mathematical necessity.
An und für sich 
I’ve wondered for a while if constructivist mathematics pedagogies may contribute to the banality of political debate. We teach our students that they can make sense of anything, rather than that they are running up against mysteries and beauties which the mind cannot fathom. So, for instance, questions regarding beauty are almost entirely absent from mathematics education literature (I believe W. M. Roth has surveyed the literature–though it may have been a related topic, I can’t find the article off hand–and I’ve done so cursorily myself.) Mathematicians, on the other hand, speak of the beauty of mathematics, and often study mathematics for its beauty (see, for instance, Serge Lang’s popular lectures on mathematics, http://bit.ly/1mKbgIJ).
Constructivist pedagogies also seem to teach (though I would need to argue it further) that learning is not something done Otherwise: there is no gift given and received, nor responses to the Face of the teacher. And thereby, have removed the richness of a meeting with Messiah from the teacher-student relationship. (Levinas said that the teacher-student relationship contains all the richness of a meeting with Messiah.)
(Which is not to say that our current pedagogies were not responding to a real problem. They were, particularly in mathematics. It seems that on older (though still modern) pedagogies, students were taught merely to submit to the teacher passively, as the teacher translated knowledge into the student; and so the teacher was not seen as a poor begger before the face of the student, and the meeting with Messiah was precluded in this demeaning treatment of the student.)
One of the most interesting distinctions between contemporary justification of action that has recourse to an external divine or transcendent power, and those of the twelfth and thirteenth centuries, is the role of interpretation and the individual concept of self.
When we look at the texts written by the of mystics from the twelfth century who took action- Hildegard von Bingen for eg. – we see that although she claims her actions are taken directly as a result of divine instruction (literally, ‘I looked, as usual, to the True Light, through which God instructed me that I was never to accede to this…’) she does so without the sense of self and subjectivity that we have, now.
It is bad enough that one could simply claim to have divine will on their side in the twelfth century when [esp.female] subjectivity wasn’t even really a proper thing, let alone a concept of the self as autonomous and individual as we have in post-modernity. Now we have no excuse.
Perhaps the most damaging situation of all is the one we find ourselves in currently, which is of wanting to still claim recourse to divine / transcendent authority (capitalist market forces) and yet *at the same time* maintain our right to autonomous, individual, wilful self-hood.