The prime example of problematic religious arguments in the pluralist public square: the appeal to authority.
If religious rhetoric or religious arguments are in some way to be disallowed from public debates about public things, this is the most common explanation for why. Public discourse has to be secular. It has to be secular in the sense that it's open to everyone. The arguments ought to be universally available, regardless of faith commitment, worldview, metaphysical picture of ultimate reality, etc. Jürgen Habermas says it has to be secular in the sense of being "post-metaphysical." This isn't supposed to mean that people with metaphysics are barred from the discourse (though sometimes it does mean that people with a minority metaphysics are excluded). It is supposed to mean, rather, that there are certain rules to public discourse. For example, that positions are not to be justified by reference to an ultimate reality not available to all. Arguments are not counted as rational if they make use of special or secret knowledge.
Appeals to a transcendent good have to be bracketted; only appeals to an immanent good are allowed.
Put another way: the argument "for the Bible tells me so" is not allowed; all arguments must be arguments based on measurable human flourishing.
In another sense, the idea is that arguments are legitimate only when they are (or can be) directed to those who disagree, and disagree in a fundamental way. A claim can only be allowed, in the discourse of pluralist democracies, if structured to persuade those who are importantly different in their metaphysical and teleological conceptions. This is to say, it is secularist.
Arguments that appeal to authority obviously, by their nature, don't do this. Thus, they're irrational, the argument goes. Or, at least, they're problematically gnostic.
A main objection to this is that it is, itself, a kind of metaphysics, snuck in under the guise of post-metaphysics. Ask why an immanent good is good, and the answer is most often an appeal to a transcendent good. The claims of human rights are not not transcendent, after all. In America, the religious right argument from Francis Schaeffer and R.J. Neuhaus to Mike Huckabee and Rick Santorum has been that that "secular" public square is not neutral. It's sometimes argued further that neutrality is incoherent and impossible. Everything is ideological, all the way down, and ideology is most especially ideological when claiming to be neutral, natural, or otherwise invisible.
There's a lot to that argument.
Another objection is that a lot of religious rhetoric and religious arguments turn out to not involve appeals to authority at all. The appeal to authority is supposed to be the religious argument par excellence. Yet, when one looks at actual instances of religious participation in public discourse in pluralist societies, the arguments aren't like that. Religious voices in the public discourse, seeking to persuade those who disagree with them in some fundamental way, most often appeal to some point of common agreement. They don't invoke the scripture or religious leaders or special revelation, for the most part. It's assumed they do, but they generally don't. They make arguments, instead, about the predicted effects of a given policy on society, and whether or not the society as a whole wants to be that kind of society.
The "religious" arguments are often -- not always, but often -- really secular. The problem of religious participation in public pluralist discourse seems, in large part, illusory.
But what about actual appeals to authority? What about the prime example of the problematic argument? Setting aside the other objections and just considering the par excellence case of the kind of speech that can't be allowed, because it's an argument that depends on access to a transcendent instead of immanent good, is it actually the case that arguments like that are like that?
To rephrase: Appeals to authority are thought to be problematic in pluralist public discourse. They're problematic, theoretically (as opposed to practically, which is a much simpler matter), because they depend on a metaphysical picture that is only accessible via special revelation, and thus exclusionary. It is thought that an appeal to authority is not a move in the discourse, it is a rejection of discourse as such. It is not an answer so much as a rejection of the possibility of the question. The appeal to authority is an appeal to a transcendent, rather than an immanent good. The argument is about what God thinks is right, e.g., rather than about what will benefit the public, resulting in human flourishing.
Is that an accurate account of how such arguments work, though?
It is at least possible to think of religious appeals to authority as argumentative short cuts. If this is the case, they're not so different from other arguments, but have been confused for something very different because of the what is left out in that short cut. Such arguments may in fact be enthymemic. That is, there's one premise that is simply unstated, that's being assumed. It could be further fleshed out and spelled out, and should be, for the sake of clarity. The premise is still implicit, though. That's not the same as it not being there. The appeal to authority could be made in a fuller way, accompanied by another claim, made explicitly. If it isn't, and is just assumed, that could be because it seemed too obvious to need articulation.
The enthymemic premise in appeals to religious authority could be that the cited authority is the best for human flourishing.
Aren't arguments, "for the Bible tells me so" actually also, for the most part, arguments "for the public good"?
The unstated premise in an ethymemic appeal to religious authority, often, is that what God thinks is right will result in human flourishing.
Charles Taylor describes an anthropocentric shift in Western thought, towards what he calls the modern moral order, which saw the "narrowing of the purposes of Divine Providence." Conceptions of God's ultimate plan, and the larger metaphysical picture, narrowed in the sense that they came to be understood as essentially the same as the public good. Taylor writes:
A crucial feature of God's order, deriving from the very way in which we discover and define it, is that it is directed to the good of the creatures which make it up, and especially ourselves. This is not new in the Judeo-Christian tradition. But it had always bent thought that God had further purposes as well in his creation; that these were largely inscrutable, but that they included our love and worship of him. So that a recognition of God and our dependence on him places immediately on us a demand which goes beyond human flourishing.
... the first anthropocentric shift comes with the eclipse of this sense of further purpose; and hence of the idea that we owe God anything further than the realization of his plan. Which means fundamentally that we own him essentially the achievement of our own good.The "eclipse" means that, at least for most religious arguments, there's an assumption that an appeal to authority is necessarily also a claim of human flourishing. While it would be clearer and likely more effective in the public sphere to articulate this more fully, it's still not correct to see such arguments as somehow entirely different. The transcendent claim about ultimate goods is not, at least in most cases, of a different species than the secularist claim about immanent good.
Supposedly immanent goods, prodded slightly with a "why?", often turn out to be transcendent. Yet, the opposite is also true. A supposedly transcendent good, when questioned, will regularly turn out to be entirely immanent, in the eclipse of the "modern moral order."
People are not as teleological and apocalyptic as is supposed, nor as post-metaphysical as they would like to imagine. The religious argument par excellence is, at least in one sense, also secular.