In modern America, that circle may not be completely shot, but it is wobbly and badly bent, according to a new landmark study conducted by the Pew Research Center.
In the meantime, almost every major branch of Christianity in the United States has lost a significant number of members, Pew found, mainly because millennials are leaving the fold. More than one-third of millennials now say they are unaffiliated with any faith, up 10 percentage points since 2007.
The alacrity of their exodus surprises even seasoned experts.
"We've known that the religiously unaffiliated has been growing for decades," said Greg Smith, Pew's associate director of religion research and the lead researcher on the new study. "But the pace at which they've continued to grow is really astounding."
It's not just millennials leaving the church. Whether married or single, rich or poor, young or old, living in the West or the Bible Belt, almost every demographic group has seen a significant drop in people who call themselves Christians, Pew found.
The drops have been deepest among two of the country's most formidable faith traditions: Catholics and mainline Protestants, so-called for their prominence in American history. At the same time, Hinduism and Islam, religions tied to recent immigrants, according to Pew, have made small but significant gains. The number of evangelicals has remained relatively steady in the past seven years, even as they decline as a percentage of the overall population.
Because the U.S. census does not ask questions about religion, Pew's survey, called "America's Changing Religious Landscape," provides one of the most reliable sources of data about the country's religious demographics. Based in Washington, Pew calls itself a nonpartisan "fact tank" and regularly produces vast and detailed studies of religion.
People who profess no faith affiliation -- often called "nones," as in "none of the above" -- now form nearly 23% percent of the country's adult population, according to the Pew study. That puts the unaffiliated nearly on par with evangelicals (25.4%) and ahead of Catholics (about 21%) and mainline Protestants (14.7%).
Seven years ago, according to Pew's previous study, the unaffiliated formed about 16% of the population, mainline Protestants were about 18%, Catholics were about 24% and evangelicals 26.3%.
Looking at the long view, the generational spans are striking. Whereas 85% of the silent generation (born 1928-1945) call themselves Christians, just 56% of today's younger millennials (born 1990-1996) do the same, even though the vast majority -- about eight in 10 -- were raised in religious homes. Each successive generation of Americans includes fewer Christians, Pew has found.
To put it simply: Older generations of Americans are not passing along the Christian faith as effectively as their forebears.
"It's not as if young people today are being raised in a way completely different from Christianity," said Smith, the Pew researcher. "But as adults they are simply dropping that part of their identity."
While Pew's study will likely cheer the hearts of atheists, the rapid rise of religiously unaffiliated Americans hasn't necessarily spawned a generation of infidels.
Just 3% of the "nones" call themselves atheists, a small bump from 2007, when 1.5% did the same. Four percent say they are agnostic, meaning they don't know if God exists, a gain of 1.6 percentage points from seven years ago.
"We are very cognizant that this does not mean there's been a straight-up spike in nonbelievers," said Paul Fidalgo, communications director for the Center for Inquiry
, a secular advocacy group. "But it's still really good news to see a whole generation of people who are making their own decisions about belief, religion and spirituality."
It's also good news for strict church-state separationists, Fidalgo said, especially those who want to see traditional religious morality disappear from debates over women's health, abortion, same-sex marriage and climate change.
While the study isn't likely to surprise many mainline Protestants, it throws their decades-long decline in membership into stark relief. Almost every American town is dotted by historic Episcopal, United Methodist, Lutheran, Presbyterian and Congregationalist churches. Increasingly, those churches are empty of young faces. Just 11% of millennials call themselves mainline Protestants. (Only 16% identify with Catholicism.)
Of America's major faiths, mainline Protestants have the worst retention rate among millennials, with just 37% staying in the fold, Pew found. By contrast, nearly two in three millennials raised without a faith continue to eschew organized religion as adults.
The collapse of American Christianity can't simply be laid at the feet of religious leaders, demographers say. There are bigger societal swings in play: Americans are marrying later, increasingly to spouses who don't share their faith, and having fewer children. (Mainline Protestants have particularly low birth rates.)
Other experts blame innovations such as the Internet and social web
, where religions can be fact-checked in real time and seekers can find communities of like-minded iconoclasts.
But Christian leaders still bear some responsibility for not connecting with younger believers, said L. Gregory Jones, a senior strategist for leadership education at Duke University in North Carolina.
Many young Christians seemed bored by church, he said, pointing to youth ministers as particularly ineffective at piquing millennials' intellectual interests. One study cited by Jones showed that nearly 70% of full-time youth ministers have no theological education.
"Christianity in the United States hasn't done a good job of engaging serious Christian reflection with young people, in ways that would be relevant to their lives."
Instead, many Christian denominations have been riven by internal struggles over homosexuality, particularly in the last decade. While most millennials back gay rights, according to separate surveys, they are more interested in working with the wider world than holding endless debates over sexual morality, Jones said.
"If it is the case that millennials are less 'atheists' than they are 'bored,' then serious engagements with Christian social innovation, and with deep intellectual reflection (and these two things are connected), would offer promising signs of hope," Jones said.