There is an important and influential stream of political thought – often associated with John Rawls and his many followers – that maintains that only secular arguments are permissible in the creation of law in a liberal polity.
I do not think that the distinction between religious and secular arguments, however, is as easy to make as champions of this tradition often hold, especially on the issues that so deeply divide us today.
Take, for example, the debate over abortion. The most sophisticated advocates on both sides typically zero in on one question: Is the prenatal human being a moral subject?
Those who defend the right to abortion – pro-choice advocates – will answer this question in the negative, but the specificity of their answer will depend on what they believe is the point in a human being’s development at which it becomes a moral subject.
The dominant view in the literature is that human beings do not become moral subjects (or persons) until sometime later in their gestation when they possess a level of organized cortical brain activity that suggests some primitive self-consciousness.
This is why pro-choice advocates will refer to fetuses prior to whichever decisive moment the advocates embrace as human beings that are potential persons but not actual persons.
Opponents of abortion – or pro-life advocates – with few exceptions, argue that the human being is a moral subject (i.e., a person) from the moment it comes into being at conception. This is because all human beings have a personal nature, even when they are not presently exercising the powers that flow from that nature’s essential properties.
These essential properties include capacities for personal expression, rational thought, and moral agency. The maturation of these capacities are perfections of a human being’s nature, and thus the human fetus can be wronged even before it can know it has been wronged.
No doubt, this view is tightly tethered to a particular reading of the Hebrew-Christian Scriptures that has been shaped by the development of a moral theology that employs the categories of certain realist philosophical traditions in its theological anthropology.
The Triumph of the Innocents by William Holman Hunt, 1876
So, there is a sense in which the pro-life argument is “religious.” But not in a way that seems relevant to the question of whether religious arguments should be excluded from the public square. Why is that?
It is because the adjective “religious” contributes nothing to our assessment of the quality of the pro-life arguments. Arguments, after all, are either sound or unsound, valid or invalid, strong or weak, cogent or non-cogent. Their premises are true or false, more or less plausible, reasonable or not.
It is not clear how labeling the pro-lifer’s case “religious,” despite its theological roots, reveals anything about its quality, or even if the argument is or is not an example of what is often called “public reason.”
Moreover, when a citizen’s argument is labeled “religious,” it seems to serve the purpose of indicating to our compatriots that the argument is sub-rational and/or without any real intellectual content.
This was made plain to me eight years ago after a lecture I gave at the law school at Texas Tech University. During the question and answer session, an audience member, a professor at the university, issued this judgment of my talk, “You’ve presented nothing but religious arguments.”
I responded, “I’m relieved. I thought you were going to say they were bad arguments.”
Even though it is not always the case, it seems generally true that when an argument is labeled “religious” it is often perceived by the target audience as equivalent to announcing that the position in question is not a deliverance of rational deliberation and/or can never serve to defeat so-called “secular arguments.”
It seems then that to suggest that the pro-life position – because it is tightly tethered to a theological tradition – should be sequestered from the public square because it rests on a “religious” argument, while the pro-choice case is perfectly acceptable because it seems to rest on a “secular” argument, that one would in fact be creating a metaphysical exclusionary rule that seems suspiciously capricious.
For what we are in fact discussing are two rival accounts on how best to answer the same question: who and what are we and can we know it?
It is not about two different subjects, religious belief and secular knowledge, the latter of which deals with reality while the former, some assert, is just a matter of opinion arising from non-rational sources.
Abortion, however, is not the only contested issue where this sort of analysis may be applied. Others include critiques of same-sex “marriage,”  physician-assisted suicide , and scientism , as well as defenses of morals legislation  and the full political participation of citizens informed by their religious beliefs .
Given this reality, we should be skeptical that attempts to distinguish between religious and secular arguments, for the sake of placing limits on the former, can advance in any meaningful way our political conversations on these contested issues.