But while the share of Muslim American adults who are converts to Islam also is about one-quarter (23%), a much smaller share of current Christians (6%) are converts. In other words, Christianity as a whole loses more people than it gains from religious switching (conversions in both directions) in the U.S., while the net effect on Islam in America is a wash.

A 2017 Pew Research Center survey of U.S. Muslims, using slightly different questions than the 2014 survey, found a similar estimate (24%) of the share of those who were raised Muslim but have left Islam. Among this group, 55% no longer identify with any religion, according to the 2017 survey. Fewer identify as Christian (22%), and an additional one-in-five (21%) identify with other faiths, or as generally “spiritual.”

One striking difference between former Muslims and those who have always been Muslim is in the share who hail from Iran. Those who have left Islam are more likely to be immigrants from Iran (22%) than those who have not switched faiths (8%). The large number of Iranian American former Muslims is the result of a spike in immigration from Iran following the Iranian Revolution of 1978 and 1979 – which included many secular Iranians seeking political refuge from the new theocratic regime.

Among those who have converted to Islam, a majority come from a Christian background. In fact, about half of all converts to Islam (53%) identified as Protestant before converting; another 20% were Catholic. And roughly one-in-five (19%) volunteered that they had no religion before converting to Islam, while smaller shares switched from Orthodox Christianity, Buddhism, Judaism or some other religion.

When asked to specify why they became Muslim, converts give a variety of reasons. About a quarter say they preferred the beliefs or teachings of Islam to those of their prior religion, while 21% say they read religious texts or studied Islam before making the decision to switch. Still others said they wanted to belong to a community (10%), that marriage or a relationship was the prime motivator (9%), that they were introduced to the faith by a friend, or that they were following a public leader (9%).

In recent years, the number of Muslim Americans has been growing steadily, by around 100,000 annually. But the fact that the shares of people who enter and leave Islam are roughly equal suggests that conversions to and from the faith are having little impact on Islam's overall growth.