Mainline churches lose more members than evangelicals and Catholics, while Catholics gain very few new believers

The new Pew survey on religion in the United States found America's unaffiliated population boomed from 16 to 23% between 2007 and 2014; the Catholic population declined from 24% to 21% and Mainline Protestants declined from 18% to 15% of the population. In contrast, evangelicals almost held steady, going from 26% to 25%, and historically Black churches, whose members generally identify as evangelical, also held steady at 7%.

Pew writes:

"While the mainline Protestant share of the population is significantly smaller today than it was in 2007, the evangelical Protestant share of the population has remained comparatively stable...As a result, evangelicals now constitute a clear majority (55%) of all U.S. Protestants."

So why are Evangelicals holding steady? I can think of two explanations:

1. Evangelicalism is more distinct from being unaffiliated; there are higher barriers to moving in and out of evangelicalism and so people are less likely to leave. The fact it's a form of Christianity that generally holds its members to stricter, more traditional standards than, at least, mainline churches, could bolster their sense of belonging and its distinctive value from being unaffiliated. Looking at mainline churches, they might ask, "If your church doesn't require anything special or distinctive from Christians, what's the point?"
2. Evangelicals lose as many members as Catholics and Mainlines, but they make up for it by gaining members, too. So, Evangelicalism's stricter standards don't seem to be holding members in; but on the other hand, evangelical churches do seem to be more capable of attracting new members to make up for the members that they along with the other churches are losing.

We can look at Pew's numbers; they published the following table:

At first glance it looks like the second explanation might be more plausible. 10% of all US adults weren't raised evangelicals, but joined an Evangelical church at some point, while 9% left an evangelical church having been raised evangelical. In contrast, mainline and Catholic churches lost even more adherents but more strikingly, only gained 2% and 6% of members between them.

But perhaps a better way to characterize each of these religious groups and how they relate to members is to measure who leaves and joins as a proportion of the church group rather than as a proportion of US adults. Fortunately that's just a little bit of simple math. Rearranging the figures on the above table, we get:

Here, the numbers change a little. Evangelical churches are similar to Catholic churches in that they've both lost around 40% of their members to other groups. Both church groups are losing fewer members than mainline churches, who lost almost 60% of the proportion of people raised in mainline churches. But evangelical churches are similar to mainline churches in that both groups pick up quite a few members from other traditions, unlike the Catholic church, which picks up very few new members. Evangelicals' combination of gaining quite a few new members and losing quite a few contrasts with mainline churches, who gain members but lose even more, and Catholic churches, which lose as many as the others and gain very few.

How does this reflect on the groups? The Catholic church seem unattractive to outsiders, either because they don't actively work so hard to convert others, have high entry barriers, or are just not appealing to people raised in other groups. Mainline churches are reasonably attractive to outsiders (more than half of whom are coming from evangelical or Catholic churches rather than unaffiliated) but lose over half of the members raised in their groups. Evangelical churches have both a moderate level of joining (principally from the other churches, not from people raised without religion) but also lose a moderate number of believers to other groups. As a whole, it seems around half of people leaving various Christian groups are not joining other Christian groups, though we can't tell from these numbers which Christian groups tend to go unaffiliated rather than to other Christian groups.

So, in summary, relative to mainline churches, it seems like conjecture (1) above is true of Evangelicals: people are less likely to leave the Evangelical church than mainline churches. But relative to the Catholic Church, (2) is truer: people do leave, but many more people join the Evangelical church compared to the Catholic Church, suggesting Evangelicalism is not better at holding on to members than the Catholic church, but is especially good at attracting new ones.

All that said, the most striking image from the graphs is the change of the unaffiliated groups. About 40% of people who are raised without religion tend to join one, a similar proportion of people who are raised evangelical or Catholic who change their affiliation. But simply because so few people were raised without religion, the proportion of people who are religiously unaffiliated now but were raised Christian is very high. Of people who aren't members of a religious group, across the ones who weren't raised in a religion and those who were, more were atheists or agnostics, and fewer said religion was even somewhat important to them.