Scott Aikin and Robert Talisse offer a clear, excellent argument for why discussion and deliberation in a democracy must be predicated on public reasons rather than exclusive, religious ones:
[T]he main function of public deliberation is not to prove that one’s views about the public good are true, but rather to show one’s fellow citizens that one’s views about the public good are justifiable. And to show one’s fellow citizens that one’s views about the public good are justifiable is to show that they are justifiable to them. In order to show that one’s views about the public good are justifiable to your fellow citizens, one must articulate the case for one’s views in terms that do not presuppose one’s own particular moral, metaphysical, or religious commitments. For your fellow citizen may reject these commitments without thereby disqualifying themselves for democratic citizenship. […]
We may say that public reasons are of the kind that cannot be dismissed as irrelevant or unintelligible by democratic citizens. Thus there is a fundamental difference between a reason such as “The Bible forbids it” and “Equality requires it.” One who dismisses the former does not thereby disqualify himself for democratic citizenship; one who dismisses the latter does. Accordingly, a group of citizens that insists on a public policy that can be supported only by means of nonpublic reasons thereby shows disrespect for their fellow citizens. Put otherwise, to affirm a public policy that cannot be supported by public reasons is in effect to say to one’s fellow citizens “Because I said so.” And that’s to deny that one’s fellow citizens are one’s equals. […]
Knowing that deliberation occurs against the backdrop of deep disagreement, we must on the one hand be willing to recognize the diversity of religious, philosophical, and ethical commitments available to democratic citizens. On the other hand, we must be able to explain the basis for any policy we advocate with reasons we can expect any of those diverse individuals to endorse as consistent with their status as a fellow free and equal citizen. That’s the tightrope of democratic justification. Democratic deliberation, then, requires us to argue from a public perspective.
I emphatically agree; the entire essay is here and worth reading.
I think it’s worth noting that Barack Obama has eloquently made this argument in the past, and it’s an argument worth making often:
Democracy demands that the religiously motivated translate their concerns into universal, rather than religion-specific, values. What do I mean by this? It requires that their proposals be subject to argument and amenable to reason. I may be opposed to abortion for religious reasons, to take one example, but if I seek to pass a law banning the practice, I can’t simply point to the teachings of my church or evoke God’s will. I have to explain why abortion violates some principle that is accessible to people of all faiths, including those with no faith at all. This is going to be difficult for some who believe in the inerrancy of the Bible, as many Evangelicals do. But in a pluralistic society, we have no choice. Politics depends on the ability to persuade each other of common aims based on a common reality.