When the Obama administration declared that employees of Catholic institutions must have contraception covered in their health plans (before the president’s deft backtracking compromise), one group of religious Americans stood firmly opposed to Obama’s original position.

No, it wasn’t Catholics. Despite the bishops’ howls of protest, poll after poll showed Catholics supporting the administration’s new rule. The opposition came from white evangelical Protestants, who stood against Obama by a whopping margin of 56 to 38.

Protestants take a stronger stand than Catholics against birth control? Curious, to say the least.

Rachel Maddow explains it by suggesting that birth control isn’t really the issue here. It’s just another convenient excuse—like controversies over Planned Parenthood and the morning-after pill—for conservatives to tar Obama as the commander-in-chief of a “war on religion.” “The right has picked a fight on this issue,” says Maddow, “because religiosity is a convenient partisan cudgel to use against Democrats in an election year.”

Okay, Rachel. But what makes religiosity convenient? If it’s all about politics, “convenient” means effective in moving voters from one column to the other.

So beneath all the fine points of the debate about contraception and health insurance, this latest brouhaha raises a much bigger question that urgently needs public discussion but rarely gets enough: In a nation supposedly built on a wall of separation between religion and politics, why are so many voters so often moved by appeals to religion?

What’s in a Wall?

The best answer I know rests on the very word “wall” and all its metaphoric meanings, as explored so insightfully by Robert Wuthnow in his analysis of “The Spirituality of Dwelling.” (It’s chapter two of his book, After Heaven.) Though the chapter deals overtly with American religious life in the 1950s, I find it a useful key to unlock the paradoxes that arise whenever religion meets conservative politics.

The gist of the argument as I read it (begging Wuthnow’s pardon for the oversimplification) is that conservatives want to live in a home and a religious community with inviolable boundaries—stout walls, both literal and metaphorical—to protect them from a world that they see filled with threatening evils. “Religion,” and everything related to it, is a prime symbol for the existence and dependability of boundaries of every kind.

That’s precisely what conservatives look for in their politics, too. For decades, psychological studies have shown that conservatives are especially afraid of uncertainty and threat, and so crave the order and structure that firm boundaries provide. As the philosopher of conservatism Michael Oakeshott put it, “To be conservative is to prefer the familiar to the unknown… the limited to the unbounded.”

As Randall Balmer, a leading scholar of evangelical Christianity, has explained it evangelicals are less concerned about specific issues than they are about feeling assured of one kind of structure in particular: “an unambiguous morality in an age of moral and ethical uncertainty”—an age that spurs evangelicals to an endless quest for obvious, immutable, and absolute contrasts between good and evil.

Politicians court these voters by saying, in as many ways as possible, “Yes, Virginia (or Iowa or Florida or Missouri) there are universal truths that can never change. You are not adrift in a sea of moral chaos. Elect me and you’re sure to have a fixed mooring to hold you and your community fast forever.”

Though Republicans have had the most success with such language in recent decades, Democrats are eagerly trying to catch up. And they have a rich tradition to draw on. Woodrow Wilson and Franklin D. Roosevelt were no more ashamed to assert their Christian faith in the public sphere than is Barack Obama.

No matter how fervently Obama professes his faith, though, conservatives insist that he is the prime symbol of what they see as the era’s all-out attack on fixed boundaries and immutable truths; hence the charge of a “war on religion.” Obama’s skin may have something to do with it; his skin so obviously portrays the blurring of what was once the nation’s most unchallengeable boundary. But any Democratic president in 2012 would have run into the same kind of political/religious attack, because that president was bound to be “the enemy.”

The historian of conservative thought Corey Robin points out that the conservative quest for limits and structure becomes, inevitably, an insistence on hierarchies. If it is safer inside the walls than outside, where danger lurks, then inside is clearly superior to outside. So, by the logic of conservatism, every boundary line separating “us” from “them” tends to become a moral line dividing the “good guys” from the “bad guys.”

If the president happens to be one of “them” (a Democrat or a liberal) he is by definition a “bad guy” regardless of his skin color, an enemy to the good and thus to religion, which is the prime symbol of all that is good.

But the demand for hierarchy has a larger impact on politics. In Bill Clinton’s often-quoted words: “When times are uncertain, the American people would rather have a leader who is wrong but strong than one who is right but weak.” Just as the father must rule to keep the home in order, and the Father in Heaven must be obeyed to keep the universe in order, so the “great white father” in the White House must have firm control to keep order in the political realm.

Which explains why, as Maureen Dowd recently put it, “Every election has the same narrative: Can the strong father protect the house from invaders?” In the logic of the conservative worldview, as Wuthnow saw, “the house” and “religion” are virtually synonymous. A leader who cannot or will not protect religion appears too weak to protect us from the dangers of the unknown in this age of uncertainty and blurred boundaries.

Those who feel plagued by the fear of uncertainty will grasp at any issue that comes along if it offers another opportunity to draw firm boundaries. If that issue concerns institutional religion, so much the better, because the language of limits and hierarchies is already prominent in that sphere. That, I suspect, is the main reason evangelical Protestants are so eager to support the Catholic bishops even when a majority of the Catholic laity don’t support their bishops.

A Conservative Quest for Absolute Truth

Every time religion collides with politics and produces controversy, it offers us a chance to discuss the overriding question of American political life: Should we use the political sphere to satisfy our desire for hierarchical structures, fixed limits, and unshakeable truths? Though conservatives show us the underlying logic of that desire most clearly, they don’t have a lock on it by any means. Those who call themselves moderates, and even many who identify as liberals, can feel a need for certainty and be tempted to use political life to satisfy it, too. So the question reaches across the political spectrum.

When it’s answered in the affirmative, people suffer: Those who want contraceptives but can’t afford them; those who want or need abortions but can’t afford them; those who want to marry but are barred by law. The list goes on.

Democracy suffers too. The essence of democracy is that we, the people, get to choose our values. We don’t discover them inscribed in the cosmos. So everything must be open to question, to debate, to change. there can be no fixed truth except that everyone has the right to offer a new view. It’s a process without end, whose outcome should never be predictable. A claim to absolute truth—any absolute truth—stops the process of democracy.

That’s the great issue lurking beneath the specifics of the latest contraception controversy. Though the legal fine points and political cost-benefit calculations of that controversy deserve the discussion they’re getting, it’s a tragedy to ignore, yet again, the essential issue that is at stake whenever religion and politics meet.