How long will it be until we begin to see a movement — egged on by activists, encouraged by receptive judges — to revoke the tax exemptions currently enjoyed by churches? A few months? A year? I don't know precisely when it will happen.

But I do know that it's going to happen.

The arguments have already begun. New York Times religion columnist Mark Oppenheimer recently made the case against such tax exemptions in a much-discussed essay for Time. We also know that the solicitor general of the United States believes that the opposition of religiously affiliated institutions to same-sex marriage is "certainly going to be an issue" going forward, since it will place them at odds with laws banning discrimination against sexual orientation. Bob Jones University resisted complying with such laws over race and saw its tax exemption revoked. The same is quite likely to happen to religious schools that continue to uphold traditional teachings about sexual morality.

Maybe such moves wouldn't be made against churches themselves. But it would be foolish to presume it couldn't or won't happen. Once the Supreme Court's Obergefell decision declaring a constitutional right to same-sex marriage is combined with laws banning discrimination against homosexuals — which already exist in well over a dozen states and are coming soon at the federal level — the case for eliminating religious tax exemptions could be powerful. And of course all of this will take place against a cultural backdrop of rapid secularization that could well produce a demographically driven sea change in the nation's attitudes toward churches and other religious institutions. Just because the United States has a long history of deference to religion doesn't mean it will continue indefinitely.

Conservatives will almost certainly oppose any effort to eliminate tax exemptions for churches. And liberals have every reason to join them in opposition.

Since 1954, section 508c of the Internal Revenue Code has stipulated that churches are automatically tax-exempt under section 503c3 as non-profit enterprises. That's why, legally, they don't have to pay federal income taxes, why those who donate money to churches are allowed to deduct those donations from their own taxable income, and why states and municipalities typically mirror these federal exemptions. The rationale behind granting such exemptions to non-profits (including churches) is that they are presumed to contribute to the common good in ways that for-profit enterprises do not, by engaging in charitable and philanthropic activities.

But it's important to recognize that the tax exemption for churches goes back much further than 1954, and even before the imposition of the permanent federal income tax in 1913. Churches have been exempt from paying taxes from the beginning of the country, and long before the law or any government agency recognized them as "non-profit" organizations. Churches were exempted because they were presumed to play the vitally important social role — a role essential to self-government — of inculcating moral virtue in citizens.

That might sound quaint to our cynical, hyper-modern ears. Some of the most secular among us may even think that this thumb on the scale in favor of encouraging faith smacks of the government "establishing" religion.

But of course the First Amendment doesn't just preclude a religious establishment. It also protects religious "free exercise," and it is on those grounds that the elimination of tax exemptions for churches should be opposed by all Americans, liberal and conservative alike.

It's true that the Constitution doesn't explicitly mandate such tax exemptions. And judged in the abstract, there's no reason to presume that religious liberty depends on them. But we don't live in the abstract. We live in a world already underway, with certain baselines, presumptions, and expectations in place.

In this real world, churches often operate on extremely tight margins, receiving the bulk of their income from voluntary donations, paying their pastors and staff a modest salary for endless hours of emotionally grueling and sometimes tedious work, and all the while performing a wide range of charitable acts within and often beyond the parish community. These churches benefit considerably, in many cases crucially, from tax exemptions for their income and property, and from the tax deductions permitted to those who contribute to their income through donations.

The removal of these exemptions would be an enormously heavy burden for many, and a catastrophic burden for some. And because this harsh, painful penalty would be imposed on churches almost entirely in response to their theologically and historically founded opposition to same-sex marriage, it would be a form of state-sponsored persecution. Religious persecution.

To which some liberals may be inclined to respond: "You bet it is — and rightly so! Why on Earth should the government be making special provisions to protect institutions that openly advocate discrimination against a category of American citizens?"

There's just one problem with this objection: It would seem to make religion as such incompatible with liberalism. All (or nearly all) religions discriminate. They divide the world into the saved and the damned, the sanctified and the sinner, the pure and the defiled, the ordered and the disordered, the righteous and the wicked, the virtuous and the vicious. That's what religions are: holistic systems of norms, practices, and beliefs that hold up some ways of living, some actions, some behaviors as better than others — with those others denounced, often in no uncertain terms.

If we forbid religions to discriminate — or empower the government to regulate how and against which behaviors a church is permitted to discriminate — we will have effectively ended religious freedom.

Are liberals really prepared to begin treating such bedrock religious tendencies — tendencies that go back to the beginnings of human culture and lie at the foundation of every civilization ever known — as beyond the bounds of acceptable thought and behavior? And to use state power to stamp them out?

There are many words that one could use to describe the impulse to do that, but "liberal" isn't one of them.