(RNS) It’s time to bury the myth that liberal theology is causing the decline of mainline churches in America -- and with it, the twin falsehood that because of their conservative creed evangelicals will own Christianity’s future.

For many years now, it’s been treated as common knowledge in some circles that the liberal beliefs of mainline churches have been the instruments of their decline. As the story goes, if you want to know why the Episcopalians, Lutherans and others like them have suffered precipitous drops in members and cultural clout since the 1960s, you need look no further than their acceptance of society’s changing sexual mores, women’s equality and so on.

Conservative churches and their strict, unbending doctrine, we’re told, are why they have held onto, and have even grown, their numbers.

How different the picture looks in this fractious summer of 2016.

As Robert P. Jones documents in his important new book, “The End of White Christian America,” white evangelicals are also shrinking now when measured as a proportion of the population.

The Southern Baptists -- the premier evangelical denomination -- have reported membership declines nine years in a row. Overall, white evangelicals have dropped from 21 percent of the population in 2008 to 17 percent in 2015. (It’s important to count white evangelicals as separate from black evangelicals, as is Jones’ practice, because the two groups are very different politically and culturally. When evangelicals of all racial/ethnic backgrounds are counted as one, their numbers add up to just over a quarter of the population.)

Don’t expect white evangelicals’ numbers to shoot back up in the coming years. Their strength appears especially anemic among young adults and those coming up behind them, as evidenced by the fact that only 10 percent of Americans under 30 are white and evangelical. That’s the same figure, by the way, as for white mainline Protestants.

“The numbers point to one undeniable conclusion: white Protestant Christians -- both mainline and evangelical -- are aging and quickly losing ground as a proportion of the population,” writes Jones, CEO of Public Religion Research Institute.

It’s interesting to juxtapose the emerging statistical reality with the rhetoric that has been common among evangelical leaders.

Prominent evangelical spokesman Albert Mohler wrote in 2005, for instance, that “doctrinal beliefs are the crucial variable determining whether churches and denominations grow or decline.”

In a different article written the same year, Mohler mocked mainline churches, as “so doctrinally confused that there is no compelling reason for anyone to join. … They reject the one way out of their crisis -- a return to biblical authority, Gospel preaching, and theological orthodoxy.”

Writing last October about evangelical rejection of changing sexual norms -- especially wider public acceptance of homosexuality -- Southern Baptist spokesman Russell Moore declared, “In a secularizing culture, churches that embrace the revolution are unnecessary.”

In view of the data, one might conclude that churches that reject the revolution are also unnecessary, at least in the minds of the younger Americans who hold the key to churches’ fates in the coming years.

Can’t conservative theology at least be credited with staving off evangelical decline for a while? Probably not, Jones says. More likely, demographic factors such as birthrates were the main reasons for evangelical strength through the '80s and '90s. Evangelicals were having more children on the whole than people in United Methodist and United Church of Christ churches and other mainline congregations.

White evangelicals have since become more upwardly mobile; more evangelical women have entered the workforce, and family sizes have tapered off.

Those who feel competitive with conservative Christians might be tempted to savor the moment. But as Jones is quick to note, this is no time for dancing on anticipated graves. If you’re a mainliner, what is there to celebrate? That fact that your church’s grim experience is now being endured by another branch of Christianity?

A victory shout might seem more in order for religiously unaffiliated Americans, who have surged to the point where they are now close to a quarter of the population and outnumber every religious category, including white evangelicals. But here, too, restraint is warranted.

For every damaging thing white evangelicals have said and done about gay rights, women’s equality and other social issues, we could probably cite equal numbers of positive contributions they and their churches have made to their communities. Will those of us in the growing secular movement be able to match their energy and commitment?

As for evangelicals themselves, it’s time to stop touting the attraction-and-retention superiority of their stout theology. If they need evidence for the beauty and truth of their doctrine, church membership numbers are no longer the place to look. They never were.

(Tom Krattenmaker is a writer specializing in religion in public life and is communications director at Yale Divinity School. He is the author of the forthcoming book "Confessions of a Secular Jesus Follower")