Defining marriage as a union of two people of the same sex is an absolute novelty. But equally serious battles over the definition of marriage have been fought before. Just think of what was at stake with Henry VIII’s marital status.

The frontiers of marriage—the limits of who may marry whom—have shifted in the past, and the shifts track tectonic changes in the foundations of the social order. Tribally organized societies welcome marriages between close cousins, and the selection of one’s spouse is likely to be, if not arranged, subject to approval by the head of the clan or household. The tribes in question need not live in huts: aristocratic families in classical and medieval times retained something of their tribal organization, including the tribal significance of marriage.

Marriages can unite tribes and kingdoms; they have military, diplomatic, and political significance. Marriage defines ingroup and outgroup, the contours of the clan itself. Children are very much the point of unions, but less because marriage happens to be a good institutional framework in which to raise the young than because marriage defines which children are legitimate members of the clan’s hierarchy and which are not. Legitimate children are the means through which the tribe—not just the individual man and woman—reproduces itself.

Christianity is not a tribal religion, and marriage as we know it has been the product of the Church battling the tribal form of social organization over centuries. The Church had to wrest control marriage away from the clan—indeed, after the fall of the Roman Empire, the Church had to rebuild civilization itself from the crude materials at hand, the blood-bound tribes that had overtaken the law-bound empire. This rebuilding required, at times, weakening the tribal loyalties of the aristocracy, and this, more than fear of inbreeding, is the reason the medieval Church began to prohibit marriage within certain degrees of consanguinity. It’s also the reason, as Francis Fukuyama has argued, why the Western Church prohibited its priests from marrying. Politics rather than sex was Rome’s concern: priests should be loyal to the Church and not enmeshed in clan intrigue.

Children again were important for the continuation of an institution beyond the immediate family: they held the future of the faith. Interfaith marriages, therefore, were strongly discouraged. Clans might find political utility in forging alliances across theological divides, but for spiritual authorities this could only wreak havoc. How was a child to be brought up if his parents were of different faiths? Jews, Christians, and Muslims could develop rules—such as the Jewish tenet, dating to Roman times, that a child is Jewish if his or her mother is—but on the whole this was a difficulty to be avoided by deprecating intermarriage.

Whether society was organized tribally or by religion, accommodations had to be made for sexual reality. True love posed challenges that mere lust did not: children arising from illicit sex could be designated as illegitimate. But what to do if a man and woman who by the standards of society should not be married were nonetheless monogamously infatuated with one another? The Church was often more sensible about these things than the clan had been; consider the case of “Romeo and Juliet.” (Even today in societies with strong kinship ties, youths may face death for making the wrong love match.)

If you were a member of a tribal society—or of the aristocratic remnant of a tribal society—one set of rules determined whom you could marry. If you lived subject above all to the governance of the Church, a different set of rules applied. In each case, the rules of marriage derived from the needs of the primary institution: the clan or the Church.

So what rules determine whom you can marry in a social order that is neither tribal nor subject to the Church?

Just as some of the tribal imperatives of marriage survived in aristocratic circles into the 20th century—that’s what “Downton Abbey” is all about—the Christian ideal of marriage survives even in a land like our own, without an established church, and in lands where established churches now play little role in shaping society. The Church couldn’t get royal families to stop marrying their cousins in the span of a thousand years. People who fear that post-Christian marriage will soon eradicate the Christian ideal have little historical grounding for their anxieties. Marriage of any particular kind may not be one of the truly permanent things, but there is clear precedent for dedicated minorities preserving their forms of marriage in the face of all adversity.

The United States from the time of its independence has explicitly discarded the old institutions that once defined marriage. The very principle of tribal hierarchy was rejected in favor of “all men are created equal”: there would be no legal privilege for aristocratic bloodlines in the United States. The American Revolution, as Robert Nisbet has pointed out, hastened the abolition of entailed estates—that is, land owned by a family rather than by an individual; again, think “Downton Abbey”—and established churches, and the U.S. Constitution did not establish any semblance of church government.

Marriage has always been a political matter, and politics in the United States was always republican and has become increasingly mass-democratic and individualistic. Even as far back as Thomas Hobbes, a political theory rooted in popular consent has been one in which every individual begins with equal and indistinguishable rights—in their politically relevant dimensions, all individuals are interchangeable. The historical tendency of democracy has been in the same direction: the franchise relentlessly expands and personal distinctions once considered relevant—property ownership, religious affiliation, sex, race—are by turns cast aside. This holds true not only for government but in society as a whole: professions and institutions once segregated by race or assigned to a single sex steadily become integrated. Conscientious exemptions may be carved out for religions, especially within their own houses of worship, and some conventional distinctions of other types remain. (Such as the preference of each sex for bathrooms of its own.) But there can be no mistaking the overall pattern.

Can it be any surprise, then, that a mass-democratic, individualistic, consumerist society in which men and women are interchangeable in all other public aspects finds them interchangeable in marriage as well?

Opponents of same-sex marriage can, of course, come up with any number of reasons why men and women should not be interchangeable where this institution is concerned. (Note, however, how rarely such opponents argue against interchangeability in other traditional capacities, such as voting.) These arguments have little prospect of gaining traction in the long run since they are contrary to the most basic inclination of a liberal democratic society—the inclination to erase distinctions between persons.

The case against gay marriage is as hard to make in a liberal-democratic society as the case for gay marriage is to make in a Christian society. In each instance, the playing field is not level: the architecture of the public order embodies certain assumptions about what a human being is and who may enforce what rules—including who may marry whom.

The fundamental question of politics is “who ultimately governs?” In some phases of Western civilization, the answer has been “the clan.” In another, it was “the Church.” Church and clan battled for supremacy for a thousand years. They both lost: once the princes seemed at last to liberate themselves from the Church after the Reformation, they soon found the new armies and administrative structures they had fostered were more deeply rooted in “the people” than in the traditional aristocracy. The people demanded to play a role greater than that of soldier and subject; they demanded sovereignty. Marriage is now what they make of it.

The paradoxical dream of Christian democrats in America and elsewhere is that the popular will, of its own accord, will conform to the teachings of the church. It won’t: interchangeable people are a multitude, possessed of a variety of opinions, none of which can legally be given weight over another. The majority rules, but the majority is an ever-shifting thing of unstable composition, and minorities who argue their case in the language of the system’s fundamental assumptions will tend steadily to win supporters. The logic of pluralism prevails in pluralistic societies over the long run for the same reason the logic of Christianity prevails in Christian societies. These social orders have already embedded their conclusions in their institutions before conscious debate has even begun, and an argument against those conclusions is necessarily an argument against the system itself.

Even as opponents of gay marriage ought to understand this, they are trapped in the very modern democratic mindset that tells them politics is a realm of endless possibility, if only the right leaders are elected and everyone works hard enough at persuading his or her fellow citizens. This mentality can be found even among those right-wingers who declare themselves staunch traditionalists and hell-bent foes of democracy. For all their protestations, they exhibit a characteristically democratic and liberal belief in the malleability of political orders. Some facts of a political order are immutable as long as that order stands—and none falls easily.

The task confronting Christians is not to transform liberal democracy into something that on principle it can never be—a simulacrum of Christendom—but to discover what place they and their faith have in a world very different from the one into which the faith came, or the one that it shaped for 1500 years.

Daniel McCarthy is editor of The American Conservative.