According to recent Pew Research findings…
Like Americans in many other religious groups, a substantial share of adults who were raised Muslim no longer identify as members of the faith. But, unlike some other faiths, Islam gains about as many converts as it loses.
About a quarter of adults who were raised Muslim (23%) no longer identify as members of the faith, roughly on par with the share of Americans who were raised Christian and no longer identify with Christianity (22%), according to a new analysis of the 2014 Religious Landscape Study. But while the share of American Muslim 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.
…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.