The ranks of the unaffiliated have been bolstered by former Christians. Nearly a quarter of people who were raised as Christian have left the group, and ex-Christians now represent 19 percent of adults.

Attrition was most substantial among mainline Protestants and Roman Catholics, who have declined in absolute numbers and as a share of the population since 2007. The acute decline in the Catholic population, which fell by roughly 3 million, is potentially a new development. Most surveys have found that the Catholic share of the population has been fairly stable over the last few decades, in no small part because it has been reinforced by migration from Latin America.

Not all religions or even Christian traditions declined so markedly. The number of evangelical Protestants dipped only slightly as a share of the population, by 1 percentage point, and actually increased in raw numbers.

Non-Christian faiths, like Judaism, Islam and Hinduism, generally held steady or increased their share of the population, reaching 5.9 percent of adults, up from 4.7 percent in 2007. Jewish adherence was steady at 1.9 percent of adults, a statistically insignificant increase of 0.2 percentage points from 1.7 percent in 2007. Adherence to Islam grew faster than any other major religious affiliation, rising by 0.5 percentage points over the last eight years, but Muslims still represent just 0.9 percent of adults in the United States.

Younger adults have been particularly likely to join the unaffiliated in recent years. In 2007, 25 percent of 18-to-26-year-olds were unaffiliated; now 34 percent of the same cohort is unaffiliated.

But the unaffiliated share of the population is increasing among older Americans as well. The Christian share of the population born before 1964 has dipped by 2 percentage points since 2007.

There are few signs that the decline in Christian America will slow. Although some might assume that young people will become more religious as they age, the Pew data gives reason to think otherwise.