(RNS) — A couple of decades ago, when young adults began showing their dissatisfaction with organized religion by voting with their feet, it was fashionable for pundits to say this was happening because those religions weren’t conservative or demanding enough.

Because the exodus was initially most pronounced among liberal, mainline Protestants like Episcopalians and Presbyterians, it was easy to point at liberalism as the root of the problem.

If churches just held fast to their standards, the thinking went, they would be fine, because strictness was what the masses secretly wanted. People craved firm boundaries. Conservatives, you will not be surprised to hear, were the most ardent supporters of this “strict churches” theory, which assured them they were already doing the most important things right.

But now the “strict churches” theory is crumbling because some strict denominations are themselves charting losses, or at least slower growth. The Southern Baptists have lost more than a million members over the last decade, according to LifeWay. Giving and attendance are down, and Baptists are seeing more gray and silver heads in the pews.

Meanwhile, the Church of Jesus Christ of Latter-day Saints has seen its once-enviable U.S. growth rate slow to under 1 percent in each of the last two years. Mormonism, which grew by just .75 percent in this country in 2017, is barely keeping pace with the growth of the U.S. population (+.71 percent).

Now it’s liberal pundits who are quick to point the finger. A HuffPost headline last year screeched that “Evangelical Christianity’s Big Turn-Off” was its relentless pursuit of a conservative political agenda. Let’s call this the “political alienation” theory, which says that churches that have waged war against LGBT rights or supported Donald Trump are reaping the fallout: Millennials want nothing to do with them.

There’s evidence to support the political alienation theory, to a point. Millennials are leaving religion in droves, and some of it is related to politics. People who vote or lean Democratic are more likely to be “nones,” defined as people who have no religious affiliation.

But that’s the key: no religious affiliation. If the political alienation theory fully explained what’s going on in American religion, millennials would be leaving conservative religions in favor of ones that are liberal and LGBT-affirming. Plenty of churches like that exist, where those on the political left would feel welcome and comfortable. But they are not growing.

Instead, folks are just leaving religion, full stop. Especially if they’re young.

So if the “strict churches” theory doesn’t explain why many conservative denominations are experiencing losses, and the “political alienation” theory doesn’t account for why people are exiting religion entirely rather than merely switching to a faith that’s more suitable, what does explain it?

No one can deny that religion has fallen on hard times. Four in 10 younger millennials (18-29) are nones, which is four times what it was in the 1980s. In fact, the fastest-growing religion in America is ... no religion at all.

The problem is that the real explanation for this should be called the “it’s complicated” theory, and complications make for crappy headlines. Everybody wants simple answers, and the answers just aren’t simple. We can give it a try, though. Here are three factors that all play a role.

1. Delayed marriage and more single adults

Pastors have joked for decades that a certain segment of people showed up to church primarily to be “hatched, matched or dispatched.” We don’t have much choice in the hatching and dispatching departments — we’re going to be born and die whether we like it or not — but what about the “matched” part? One of the biggest demographic trends of our time is that millennials are delaying marriage or not getting married at all. And since there’s a strong correlation between being married and being involved in religion, the fact that fewer Americans are getting married is worrisome news to clergy.

2. Fertility

The number of children a family has is related to the couple's religious involvement — couples without kids are a bit less likely to be religious. So the fact that fertility is on the decline is, again, worrisome news for organized religion. (To say nothing of the long-term implications for things like Social Security.)

3. The growth of the nones

This seems like a circular argument: The nones are growing because the nones are growing! But sociologically, it makes a kind of sense. Some nonbelievers might have stayed in organized religion in previous generations just because it was socially expected, and there were consequences for not joining the religious crowd. The numeric growth of the nones has removed some of those barriers, so that other closeted nones feel more comfortable leaving religion too. There is an infrastructure and support system for them.

So the next time someone tells you, “Millennials are leaving religion because evangelicals are in love with Donald Trump,” you can tell them they’re partly right. Political frustration is a factor. But ... the truth is also much, much more complicated.

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