(CNN) -- If you're the parent of a Christian teenager, Kenda Creasy Dean has this warning:

Your child is following a "mutant" form of Christianity, and you may be responsible.

Dean says more American teenagers are embracing what she calls "moralistic therapeutic deism." Translation: It's a watered-down faith that portrays God as a "divine therapist" whose chief goal is to boost people's self-esteem.

Dean is a minister, a professor at Princeton Theological Seminary and the author of "Almost Christian," a new book that argues that many parents and pastors are unwittingly passing on this self-serving strain of Christianity.

She says this "imposter'' faith is one reason teenagers abandon churches.

"If this is the God they're seeing in church, they are right to leave us in the dust," Dean says. "Churches don't give them enough to be passionate about."

What traits passionate teens share

Dean drew her conclusions from what she calls one of the most depressing summers of her life. She interviewed teens about their faith after helping conduct research for a controversial study called the National Study of Youth and Religion.

The study, which included in-depth interviews with at least 3,300 American teenagers between 13 and 17, found that most American teens who called themselves Christian were indifferent and inarticulate about their faith.

The study included Christians of all stripes -- from Catholics to Protestants of both conservative and liberal denominations. Though three out of four American teenagers claim to be Christian, fewer than half practice their faith, only half deem it important, and most can't talk coherently about their beliefs, the study found.

Many teenagers thought that God simply wanted them to feel good and do good -- what the study's researchers called "moralistic therapeutic deism."

Some critics told Dean that most teenagers can't talk coherently about any deep subject, but Dean says abundant research shows that's not true.

"They have a lot to say," Dean says. "They can talk about money, sex and their family relationships with nuance. Most people who work with teenagers know that they are not naturally inarticulate."

In "Almost Christian," Dean talks to the teens who are articulate about their faith. Most come from Mormon and evangelical churches, which tend to do a better job of instilling religious passion in teens, she says.

No matter their background, Dean says committed Christian teens share four traits: They have a personal story about God they can share, a deep connection to a faith community, a sense of purpose and a sense of hope about their future.

"There are countless studies that show that religious teenagers do better in school, have better relationships with their parents and engage in less high-risk behavior," she says. "They do a lot of things that parents pray for."

Dean, a United Methodist Church minister who says parents are the most important influence on their children's faith, places the ultimate blame for teens' religious apathy on adults.

Some adults don't expect much from youth pastors. They simply want them to keep their children off drugs and away from premarital sex.

Others practice a "gospel of niceness," where faith is simply doing good and not ruffling feathers. The Christian call to take risks, witness and sacrifice for others is muted, she says.

"If teenagers lack an articulate faith, it may be because the faith we show them is too spineless to merit much in the way of conversation," wrote Dean, a professor of youth and church culture at Princeton Theological Seminary.

More teens may be drifting away from conventional Christianity. But their desire to help others has not diminished, another author says.

Barbara A. Lewis, author of "The Teen Guide to Global Action," says Dean is right -- more teens are embracing a nebulous belief in God.

Yet there's been an "explosion" in youth service since 1995 that Lewis attributes to more schools emphasizing community service.

Teens that are less religious aren't automatically less compassionate, she says.

"I see an increase in youth passion to make the world a better place," she says. "I see young people reaching out to solve problems. They're not waiting for adults."

What religious teens say about their peers

Elizabeth Corrie meets some of these idealistic teens every summer. She has taken on the book's central challenge: instilling religious passion in teens.

Corrie, who once taught high school religion, now directs a program called YTI -- the Youth Theological Initiative at Emory University in Georgia.

YTI operates like a theological boot camp for teens. At least 36 rising high school juniors and seniors from across the country gather for three weeks of Christian training. They worship together, take pilgrimages to varying religious communities and participate in community projects.

Corrie says she sees no shortage of teenagers who want to be inspired and make the world better. But the Christianity some are taught doesn't inspire them "to change anything that's broken in the world."

Teens want to be challenged; they want their tough questions taken on, she says.

"We think that they want cake, but they actually want steak and potatoes, and we keep giving them cake," Corrie says.

David Wheaton, an Atlanta high school senior, says many of his peers aren't excited about Christianity because they don't see the payoff.

"If they can't see benefits immediately, they stay away from it," Wheaton says. "They don't want to make sacrifices."

How 'radical' parents instill religious passion in their children

Churches, not just parents, share some of the blame for teens' religious apathy as well, says Corrie, the Emory professor.

She says pastors often preach a safe message that can bring in the largest number of congregants. The result: more people and yawning in the pews.

"If your church can't survive without a certain number of members pledging, you might not want to preach a message that might make people mad," Corrie says. "We can all agree that we should all be good and that God rewards those who are nice."

Corrie, echoing the author of "Almost Christian," says the gospel of niceness can't teach teens how to confront tragedy.

"It can't bear the weight of deeper questions: Why are my parents getting a divorce? Why did my best friend commit suicide? Why, in this economy, can't I get the good job I was promised if I was a good kid?"

What can a parent do then?

Get "radical," Dean says.

She says parents who perform one act of radical faith in front of their children convey more than a multitude of sermons and mission trips.

A parent's radical act of faith could involve something as simple as spending a summer in Bolivia working on an agricultural renewal project or turning down a more lucrative job offer to stay at a struggling church, Dean says.

But it's not enough to be radical -- parents must explain "this is how Christians live," she says.

"If you don't say you're doing it because of your faith, kids are going to say my parents are really nice people," Dean says. "It doesn't register that faith is supposed to make you live differently unless parents help their kids connect the dots."

'They called when all the cards stopped'

Anne Havard, an Atlanta teenager, might be considered radical. She's a teen whose faith appears to be on fire.

Havard, who participated in the Emory program, bubbles over with energy when she talks about possibly teaching theology in the future and quotes heavy-duty scholars such as theologian Karl Barth.

She's so fired up about her faith that after one question, Havard goes on a five-minute tear before stopping and chuckling: "Sorry, I just talked a long time."

Havard says her faith has been nurtured by what Dean, the "Almost Christian" author, would call a significant faith community.

In 2006, Havard lost her father to a rare form of cancer. Then she lost one of her best friends -- a young woman in the prime of life -- to cancer as well. Her church and her pastor stepped in, she says.

"They called when all the cards stopped," she says.

When asked how her faith held up after losing her father and friend, Havard didn't fumble for words like some of the teens in "Almost Christian."

She says God spoke the most to her when she felt alone -- as Jesus must have felt on the cross.

"When Jesus was on the cross crying out, 'My God, why have you forsaken me?' Jesus was part of God,'' she says. "Then God knows what it means to doubt.

"It's OK to be in a storm, to be in a doubt," she says, "because God was there, too."