A recent report by the Pew Research Center has uncovered a striking fact: Nine percent of Americans say that they are Catholics, but that Catholicism is not their religion.

Two out of 10 respondents told the Pew pollsters that they are Catholics in the sense that they “claim the faith as their current religion.” Another one out of 10 “were raised in the faith and have now fallen away.” But the third group, the 9 percent that Pew calls “cultural catholics,” are more of a puzzle. They do not claim Catholicism as their religion since they are Protestants, atheists, agnostics, or have no religious affiliation. Yet they also regard themselves as “indelibly Catholic by culture, ancestry, ethnicity, or family tradition.” If Catholicism is a religion, then why do so many Americans call themselves Catholics and yet do not have a Catholic religious identity?

There is a basic assumption about religion at work in the claims cultural Catholics make about their identity. Even though about 13 percent of them occasionally attend Mass, they do not consider that practice sufficient for them to claim Catholicism as their religion. Instead they say they are Catholic “because of their Catholic background,” which mostly means that they were raised in Catholicism as children. They feel they have inherited a Catholic identity, but have made a conscious choice not to embrace Catholicism as their religion.

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.

Today, according to another Pew study, nearly half of all Americans have changed their religion at one time or another, many of them more than once. In the contemporary United States, Catholics are especially prone to switching religions. (Cultural Catholics are, by definition, people who have changed their religion.) Catholicism lost large numbers of adherents in the wake of the clergy sex-abuse scandals. Even people who remain in the faith in which they were raised must acknowledge that there are other religious options available to them, including the option of no religion at all.

Plenty of Americans have picked their religion, and so they think of religion as something to be picked. Americans have developed this distinction, which would not have made sense centuries ago. As the Pew report shows, American Catholics hold onto the identity that they have inherited, but they don’t think of that identity as religious unless they have chosen it for themselves.