Trying to form a Christian political philosophy in a day and age where secular, liberal democracy is the dominant form of government has a central problem, how the sacred and the profane ought to interact, or even if they ought to interact at all. This central problem has two sides for the Christian. What ought to be the political response of the Christian to living in a liberal, secular democracy? What ought be the response of a secular, liberal democracy response to religion? The first of these two questions is mostly interesting only to Christians as, presumably, neither secularists nor adherents of other faiths are all that intrigued by the question. The other side of the question, however, is one that has been grappled with for most of the modern era and is presently being grappled with throughout the entire world with a range of approaches. Some regimes have outright persecuted all religions. Others have warmly embraced at least some religions. Still others have simply attempted to ignore religion or to excise religion from public discourse with regards to politics. This latter approach, which various groups have argued ought to be the case in the US, I think is somewhat misguided. While there are certainly forms of religious dialog (dialectics and dogmatics) that are troubling with regards to making public policy in a secular society, there is a third way of expressing religion (the demonstration of religious truths through analytic reason) that ought to be warmly welcomed.
One of the most useful categorizations of religious dialog I have come across is the traditional Islamic tripartite division of religions instruction followed by Abu Nasr al-Farabi: political science (siyāsah), dogmatic theology (fiqh, which is more frequently translated as `jurisprudence', but in context, I think `dogmatic theology' fits better), and dialectic (kalām). The first of these, political science, consists of arguments based on that which is evident, self-evident, or the conclusions of other arguments based on the same. The second, dogmatic theology, consists of starting with the pronouncements of prophets or other inerrant teachers as indisputable premises to discover what implications can be derived from them. The last of these, dialectic, consists of convincing infidels, heathens, and heretics of the truth of the indisputable premises of the prophets by any means necessary. (See Najjar's translation of Farabi's Enumeration of the Sciences in Lerner and Mahdi's Medieval Political Philosophy.)
I would think that it would be relatively uncontroversial to suggest that both dogmatic theology and dialectic cannot be an acceptable basis for the rule of law in a liberal, secular democracy as they both begin with the revealed knowledge of religion. The difference between dialectic and dogmatics is not the foundation of the discussion, but the methods used to build on that foundation. Dogmatics uses logic and reason to fully develop revealed truths, most commonly so that the practitioners of a religion can understand what their faith has to say about a particular situation that is not explicitly covered within their body of revealed knowledge. Dialectics, however, may or may not be built on logic and reason. It is the practice of taking anything that seems useful to make it seem like the starting point of revealed religion is a conclusion reached by knowledge outside of the faith. This can take several forms: using religious teachings from outside of the faith, such as Saint Paul's appeal to the statue built to the unknown god at the Athenian agora in the Acts of the Apostles; the attempt to make teachings outside of the faith seem absurd, such as religious anti-evolutionists who argue that the theory of evolution says we're all monkeys; or even outright dishonesty. While these methods are fine for religious instruction within a religious movement and even for dialog between religious movements, it seems to me almost self-evident that from the point of view of a secular state, that they are not a viable foundation on which to build public policy.
But this leaves what al-Farabi called political science, truths which have as their basis, principles that are evident, self-evident or demonstrations reached according to premises which are evident or self-evident. This point does not seem to me to be particularly controversial. Clearly, one cannot imagine much of a valid secular objection to forging public policy on the basis of that which is evident or self-evident. There may be objections as to which particular ideas are self-evident or of whether or not the evidence actually supports a particular premise, but the framework on which this type of knowledge sits is well accepted by secularism. Some would go so far as to argue that this framework is the very foundation of secularism. The only serious objection likely to be raised is not that this is a bad foundation for public policy but that this type of knowledge can not properly be categorized as religious knowledge at all.
This objection that demonstrative truths cannot be religious, however, is one that is imposed on religion rather than one that comes from within religion. While it is true that there have been some religious voices that have argued that there is nothing reasonable about religion (Tertullian's infamous ``I believe because it's absurd'' comes to mind), these voices are in the minority. From the early Christian apologists through the medieval era in both eastern and western Christendom, the prevailing view was that reason and faith were complimentary rather than being opposed. Even Tertullian, the third century bishop whose answer to the question `What does Athens have to do with Jerusalem?' was `nothing,' extensively used logic and reason in his writings. To say that he rejected all reason as being opposed to faith is to make his actual beliefs into a straw-man. And, in fact, the oft cited phrase ``I believe because it is absurd is most likely a very bad translation of a phrase that was making the opposite point, that some things are so absurd that those saying they actually happened must have really observed them because, on the face of it, they are so absurd as to be unworthy of belief unless they were eyewitnessed. The view that reason and faith are mutually exclusive is a relatively modern construct, only becoming relatively widespread after the Reformation and Enlightenment, and one that only a minority of religious adherents believe however much some secularists might wish to pin this view on all religionists.
But might it not be the case that a softer form of that same objection might make more sense? Even if it is true that some religious teachings consist of that which is evident and self-evident, why should a secular state even bother paying attention to those conclusions about policy that it can reach on its own without recourse to religion? On the face of it, this seems to me to be a much stronger objection as I would certainly concede that the truths of religion which are capable at being arrived at entirely through analytic reason and the evidence of our senses do not need to be presented by the adherents of a particular religion in order for secularists to discover them. That said, history illustrates many situations where various religious movements led the way on issues most secularists now take for granted. From the abolition of slavery throughout the British Empire in eighteenth and nineteenth centuries to the civil rights movement in the US in the middle of the twentieth century, various religious movements have pushed mostly secular states to accept what are now widely considered to be indisputable truths. In these cases the secular society in question was capable of reasoning to the same conclusion on its own but left to its own devices had not. It may very well be the case that society may have eventually come to these conclusions without the involvement of religion, but it is also almost indisputably the case that these changes would not have come about in history when they did were not for the involvement of many who were convinced that they were doing God's work.
Consequently, it seems to me that, from the point of a secular, liberal democracy, there not only can be room to allow for public policy based on religious convictions but history shows us that from time to time, such policy furthers the interests of a secular society in a positive fashion. This is not to say, however, that all religious convictions are equally valid for inclusion in the public discourse over matters of political policy. Clearly those religious convictions which cannot be communicated by relying on evident and self-evident truths have little room in the sphere of public discourse within a secular regime. Hence, the role played by religious convictions in a secular, liberal democratic regime is strictly circumscribed. The task of the believer in a particular faith who would influence public policy, then, is to work towards grounding the religious beliefs of his or her tradition in premises which are acceptable to all the members of society and not only those who share those specific religious beliefs. In doing so, the believer can serve as a tremendous moral influence with regards to public policy in a modern liberal and secular democracy.