When asked what it means to be a Catholic, some people say that it is “a matter of religion,” others that it is a matter of “ancestry or culture.” Religion and religious identity are seen as distinct from the cultural identity. It is not simply an assemblage of beliefs and practices, but the fact that one has chosen to believe and practice, that marks something as religious to Americans. One basic assumption that Americans make about religion, then, is that it is something they actively choose, not something that they simply inherit.

The idea that people must pick their religion is by no means natural. Americans came to think of religion as a matter of choice because of a long historical process. When religion was disestablished in the aftermath of the American Revolution, the removal of state encouragement for religion had the effect of encouraging people to decide matters of religion for themselves.

The 19th-century expansion of evangelicalism into the Protestant mainstream intensified conversion as a live option, as evangelists preached revivals and traveling pitchmen brought tracts and Bibles to people’s doors. Irreligion, whether in the occasional growth of real atheists or agnostics or in the constant fears of imagined “infidels,” also played an important role.

American Christians came to define themselves in contrast to irreligion, and thought of their children as having to exercise a deliberate choice to become Christian, rather than simply being Christian by baptism or upbringing. (The recent attention to new prevalence of religious “nones,” meaning people without a religious affiliation, misses the fact that for much of the 19th century many people were "nones," only to be slowly missionized.)

Many religions have practices for passing religious identities on to children deeply embedded within them. Evangelicalism, by contrast, has long embraced the notion that everyone must choose their own religion. People in other American religions have had a difficult time resisting the operating assumption of the American mainstream. They have had to deal with, or have come to embrace, the assumption that religion is a choice.

Religions which, rightly or wrongly, are considered to be strongly defined by ethnicity have been more likely to escape the notion of religion as choice. Nineteenth-century Catholicism was often strongly marked as an ethnic faith by Protestant anti-Catholicism. But even then, Catholics loved to compile lists of “distinguished” and not-so-distinguished converts, emphasizing the voluntary nature of their religious identity. Cultural Catholics today, who think of religious identity as a matter of choice, take that a step further. There is a similar phenomenon among American Jews, some 22 percent of whom (32 percent among Millennials) think of themselves as Jewish—but not by religion. It may be that, in time, religious communities that are relatively new to the United States such as Muslims and Hindus will be subject to the same pressure, and similarly come to think of their faiths more as a choice than as an inheritance.