LifeWay Research found seven in 10 Protestants ages 18 to 30 who went to church regularly in high school quit attending by age 23. A third of those had not returned by age 30. That means about one-fourth of young Protestants have left the church.
The Barna Group says six in 10 young people will leave the church permanently or for an extended period starting at age 15.
The 2012 Millennial Values Survey, conducted jointly by the Public Religion Research Institute and Georgetown University's Berkley Center for Religion, Peace and World Affairs, found college-age millennials are 30 percent more likely than the general population to be religiously unaffiliated.
Millennials report lower levels of religious engagement across the board. Just one in four says he or she attends religious services at least once a week, while 43 percent say they seldom or never attend.
Nearly half of younger millennials still live with their parents, but those who live at home are no more likely to attend church than those who do not.
Experts say the trend away from organized religion dates back to the early 1990s. While there's nothing new about young adults drifting from the faith after they leave home until they marry and have children of their own, pollsters fear current trends signal more than sowing wild oats.
Millennials are leaving the faith at higher rates than ever before, and with many postponing life-changing events like career and marriage to later ages, more and more young adults are making choices with sometimes lifelong consequences largely devoid of religious influence.
Experts point to various reasons for the exodus. Conventional wisdom attributes the trend to moral compromise. Free from parental control, young adults adopt lifestyles they were taught were sinful. Unwilling to change and desiring to avoid feelings of guilt, they drop their faith commitment.
While that certainly is a factor, researchers suggest the picture is more complicated.
According to a 2010 Christianity Today article by Drew Dyck, many young people influenced by college professors and writings of the rash of New Atheist authors consider religion intellectually inferior to academic study.
Others have postmodern misgivings about hyper-logical apologetics such as C.S. Lewis' Mere Christianity that were persuasive to earlier generations of young skeptics.
Some have been hurt by Christians, such as victims of sexual abuse by clergy, and then doubly victimized when other church members ostracize and judge them unforgiving. Christa Brown, a victims' advocate and author of This Little Light: Beyond a Baptist Preacher Predator and His Gang, calls clergy sex abuse a "soul-murdering" offense, because it robs many formerly devout victims of the desire to have anything to do with the church.
For most, however, research suggests the main reason for disengaging from religion is far less dramatic. Their faith simply does not seem relevant or important to their daily lives.
Studies find younger millennials have mixed feelings about Christianity. Most believe present-day Christianity has good values and principles and that it consistently shows love toward other people. At the same time, majorities view Christians as hypocritical, judgmental and anti-gay.
Thom Rainer, president of LifeWay Christian Resources, who has done extensive research about church dropouts, says contrary to what most people think, the unchurched aren't bothered all that much by some hypocrisy among Christians, because they recognize nobody is perfect. They are turned off, however, by Christians who treat other Christians poorly, talk but don't listen and harbor holier-than-thou attitudes.
Rainer found a common theme running through the excuses why young adults stop attending church. "Stated simply, they just did not see that church was essential to their lives," he concluded.
Some observers say churches wringing their hands over young people leaving the fold have only themselves to blame.
Dyck, author of Generation Ex-Christian: Why Young Adults Are Leaving the Faith and How to Bring Them Back, says over the last couple of decades the focus in youth ministry has shifted from spiritual growth to attracting large numbers of kids and keeping them entertained. That, he says, has produced unintended consequences. Many youth ministries have become largely devoid of spiritual engagement.
"Too many youth groups are holding tanks with pizza," LifeWay Research director Ed Stetzer said in a 2007 interview with USA Today. "There's no life transformation taking place. People are looking for a faith that can change them and to be a part of changing the world."
Kara Powell, executive director of the Fuller Youth Institute and co-author of Sticky Faith: Everyday Ideas to Build Lasting Faith in Your Kids, says as youth ministry became more professionalized in the last 50 years, kids wound up being segregated from the rest of the church. While there is a place for age groups to be together with their own kind, she said in a Relevant Magazine interview, having intergenerational relationships is vitally important in high school and college students' development of a mature faith.
"The original churches in the first century were multigenerational, were multi-ethnic," she said. "We need to provide space for folks in similar life spaces to chat and share community, but balance is something we swing through on our way to the other extreme."
Experts say even with today's sobering numbers, the news isn't all bad.
Many formerly churched millennials say they harbor no ill will toward Christianity and see an open door for their possible return to the fold.
In Essential Church, Thom and Sam Rainer said the "dechurched" often need nothing more than a gentle nudge from family or friends to reconnect. Four in 10 "rechurched" individuals in their study said parents or other family members were instrumental in their return to church.
Another two out of 10 said they returned to church with encouragement from friends or acquaintances.