Christianity will almost certainly still be the predominant religion in the United States in 35 years, but its place in the firmament of American theism won't be quite as high. Atheism and its cousin agnosticism, on the other hand, are due for a pretty sizable step up, according to projections from the Pew Research Center's Forum on Religion & Public Life.

That may not be very surprising, but the reasons for the shifting demography of American religion are pretty interesting. There will be more Christians in 2050 than today, Pew predicts, and Christians will continue to be more fertile than the religiously unaffiliated, Pew's term for "atheists, agnostics, and people who do not identify with any particular religion." Currently, Christians have a fertility rate of 2.1 children per couple, just at the replacement level, versus 1.6 for the unaffiliated.

But thanks primarily to people switching religions or dropping out, the percentage of the U.S. that identifies as Christian will decline to 66.4 percent in 2050, from 78.3 percent. Meanwhile, the religiously unaffiliated population will increase to 25.6 percent, from 16.4 percent.

Numerically, the Christian population will rise slowly then flatten, adding a total of about 19,000,000 adherents between 2010 and 2050. The ranks of the religiously unaffiliated will nearly double, growing by nearly 50,000,000. Here's what that looks like in a chart:

It's worth noting that most of the unaffiliated aren't atheist or even agnostic, according to a 2012 report by Pew and PBS. In that survey, 13.9 percent identified as "nothing in particular," versus 3.3 percent who called themselves agnostic and 2.4 percent who self-identified as atheist. And two-thirds of the religiously unaffiliated said they believe in God.

Nonetheless, the shift away from Christianity toward "nothing in particular" has significant implications for how America lives, gives, and votes, among other things. A December 2013 Gallup poll, for example, found that more Christians (84 percent) say they give money to charity than people with no religion (77 percent).

Politically, the rise of the "nones" could spell trouble for the Republican Party. There's been a lot of discussion about how the growing Hispanic population — projected by the U.S. Census Bureau to increase by 86 percent between 2015 and 2050, to 112 million, or about 28 percent of the U.S. populace — will help Democrats (unless Republicans make inroads with Latinos).

But the religiously nonaffiliated also skew to the left. Pew's 2012 study found that 63 percent of the "nones" identified as Democratic or leaning toward the Democratic Party, and they were roughly twice as likely to call themselves liberal as conservative. If the religiously unaffiliated become a political force, Republicans will either have to capture more of the religious vote or court the religiously noncommittal.

The political party seen as more friendly toward Muslims could benefit from demographic shifts, too, as Islam is projected to be the No. 3 U.S. religion by 2050, passing Judaism both numerically and percentage-wise:

Muslims had the highest U.S. fertility rate in the last five years — 2.7 kids per couple — but Pew's predictions about the surge in Islam, Buddhism, and Hinduism are tied largely to immigration from the Asia-Pacific, Middle East, and North Africa. In fact, in recent years, Asians have made up the the largest slice of immigrants to the United States, according to Census data.

Pew can't read the future, of course, but if its educated guess about the coming demographic shifts is anywhere close to accurate, religion in America will look pretty different in 35 years, in ways big and small. Churches, mosques, synagogues, ashrams, temples, and politicians should all plan accordingly.