It’s relatively well-known that religion in this country is declining, with 26% of Americans now describing themselves as nonreligious (9% adorning the atheist or agnostic label, 17% saying they are “nothing in particular”). Less discussed is where these growing numbers come from and just how much “faith switching” happens here.

For example, about 20% of citizens are former Christians, one in every five people you pass on the street. Where these individuals go isn’t a foregone conclusion — at times it’s to Islam (77% of new converts used to be Christians), Hinduism, or other faiths (“Members of non-Christian religions also have grown modestly as a share of the adult population,” the Pew Research Center reports). But mostly it’s to the “none” category, which has thus risen dramatically and is the fastest-growing affiliation. In a majority-Christian country that is rapidly secularizing, all this makes sense. (For context, 34% of Americans — one in three people — have abandoned the belief system in which they were raised, this group including atheists, Christians, Buddhists, Muslims, everyone. 4% of Americans used to be nonreligious but are now people of faith.)

While Islam is able to gain new converts at about the same rate it loses members, thus keeping Islam’s numbers steady (similar to Hinduism and Judaism), Christianity loses far more adherents than it brings in, and is therefore seeing a significant decline (77% to 65% of Americans in just 10 years):

19.2% of all adults…no longer identify with Christianity. Far fewer Americans (4.2% of all adults) have converted to Christianity after having been raised in another faith or with no religious affiliation. Overall, there are more than four former Christians for every convert to Christianity.

This statistic holds true for all religions, as well: “For every person who has left the unaffiliated and now identifies with a religious group more than four people have joined the ranks of the religious ‘nones.'”

This is so even though kids raised to be unaffiliated are somewhat less likely to remain unaffiliated! 53% of Americans raised nonreligious remain so. This is better than the 45% of mainstream protestants who stick with their beliefs, but worse than the 59% of Catholics or 65% of evangelical protestants. (Hinduism, Islam, and Judaism again beat everyone — one shouldn’t argue that high retention rates, or big numbers, prove beliefs true, nor low ones false.) Yet it is simply the case that there are currently many more religious people to change their minds than there are skeptics to change theirs:

The low retention rate of the religiously unaffiliated may seem paradoxical, since they ultimately obtain bigger gains through religious switching than any other tradition. Despite the fact that nearly half of those raised unaffiliated wind up identifying with a religion as adults, “nones” are able to grow through religious switching because people switching into the unaffiliated category far outnumber those leaving the category.

Overall, this knowledge is valuable because the growing numbers of atheists, agnostics, and the unaffiliated are occasionally seen as coming out of nowhere, rather than out of Christianity itself. (And out of other faiths, to far lesser degrees: Muslims are 1% of the population, Jews 2%.) As if a few dangerous, free-thinking families were suddenly having drastically more children, or a massive influx of atheistic immigrants was pouring into the U.S., skewing the percentages. Rather, the 26% of Americans who are nonreligious is comprised of much of the 20% of Americans who have abandoned Christianity. The call’s coming from inside the church.

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