The Christian share of adults in the United States has declined sharply since 2007, affecting nearly all major Christian traditions and denominations, and crossing age, race, and region, according to an extensive survey by the Pew Research Center.
Seventy-one percent of American adults were Christian in 2014, the lowest estimate from any sizable survey to date, and a decline of 5 million adults and 7 percentage points since a similar Pew survey in 2007.
In Massachusetts, Catholics still constitute the largest Christian denomination at 34 percent, but the unaffiliated cohort is right behind, at 33 percent. The share of Massachusetts Catholics dropped 9 percentage points since the last survey in 2007, while the unaffiliated grew 16 percent.
The Christian share of the population has been declining for decades, but the pace rivals or even exceeds that of the country’s most significant demographic trends, like the growing Hispanic population. It is not confined to the coasts, the cities, the young, or other liberal and more secular groups.
“The decline is taking place in every region of the country, including the Bible Belt,” said Alan Cooperman, the director of religion research at the Pew Research Center and the lead editor of the report.
The decline has been propelled in part by generational change, as relatively non-Christian millennials reach adulthood and gradually replace the oldest and most Christian adults. But it is also because many former Christians, of all ages, have joined the rapidly growing ranks of the religiously unaffiliated or “nones”: a broad category including atheists, agnostics, and those who adhere to “nothing in particular.”
The report does not offer an explanation for the decline of the Christian population, but the low levels of Christian affiliation among the young, well educated, and affluent are consistent with theories for the rise of the unaffiliated, like the politicization of religion by American conservatives, a broader disengagement from all traditional institutions and labels, the combination of delayed and interreligious marriage, and economic development.
Overall, the religiously unaffiliated number 56 million and represent 23 percent of adults, up from 36 million and 16 percent in 2007, Pew estimates. Nearly half of the growth was from atheists and agnostics, whose tallies nearly doubled to 7 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, 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.
Non-Christian faiths, like Judaism, Islam, and Hinduism, generally held steady or increased their share of the population, reaching 5.9 percent of adults. Jewish adherence was steady at 1.9 percent of adults. Islam grew faster than any other major affiliation, rising by 0.5 percentage points over eight years, but Muslims are just 0.9 percent of adults.
Michael O’Loughlin of the Globe staff contributed to this report.