“They’re better at holding on to the people born into their tradition (65 percent retention compared to 59 percent for Catholics and 45 percent for Mainline Protestants), and they’re a stronger attractor for people leaving other faiths. According to Pew’s data on conversion rates, 10 percent of people raised Catholic wind up as evangelicals. Just 2 percent of people born as evangelicals wind up Catholic.”

Religions in the United States grow and shrink because of immigration, births, deaths and many other factors, but a recent report from the Pew Research Center offers a chance to look in detail at which religions are gaining and losing members the old-fashioned way: by recruiting new members.

Pew took a look at which religions have gained and lost followers when people change faiths, and we used its data to run the tape forward and see which religions would be on the rise if people kept switching at the current rates. This model isn’t a projection of the future (because it doesn’t take into account other factors); instead, it’s a way to measure the pull that religions are exerting on potential converts at present, by taking today’s trends to their logical conclusions.

In its 2014 Religious Landscape study, which surveyed more than 35,000 Americans, Pew asked respondents about their current religious practice and the tradition in which they were raised. Pew found that the group of Americans who are unaffiliated with any religion gained the most members, on net, from religious churn. Although only 9.2 percent of Americans reported being born into a religiously unaffiliated household, 22.8 percent described themselves as unaffiliated by adulthood. The share of Americans who are Christians, as a whole, dropped 15.0 percentage points (from 85.6 percent at birth to 70.6 percent at present). Pew highlighted some of the most dramatic swings in the table below but also collected data on seven additional denominations.

The numbers presented by Pew help describe current patterns, but they don’t tell the whole story of which denominations are most attractive to people who might be looking for a new one. Gain and loss numbers can wind up skewed by how large (or small) different groups are to start off with. It’s easier for unaffiliateds to gain followers, on net, because there are fewer unaffiliateds to lose in the first place.

In fact, people raised unaffiliated have about the same chance of staying unaffiliated (53 percent) as people raised Catholic do of staying Catholic (59 percent), according to the Pew data. But when unaffiliateds lost about half of their 9.2 percent share of U.S. adults, they lost only 4.3 percentage points’ worth of the population at large. In contrast, because nearly a third of Americans are raised Catholic, when that church loses about half its members, it amounts to a much bigger group of people.

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