At Rooftop Church, just outside St. Louis, Missouri, it's not collars and frocks ? it's baseball caps and jeans.

Head pastor Matt Herndon sets the tone on Sundays.

"When a lot of people come in, they do notice some things that maybe they wouldn't see at other churches, [like] oh that's strange, he's wearing a hat. Oh, we just watched a video clip from 'Beauty and the Beast,'" Herndon said. "We really do want to engage with people in a way that they can understand and lean into."

Rooftop is one of an estimated 40,000 nondenominational Christian churches in the U.S., meaning its teachings are rooted in the Bible, but it's an independent house of worship. What Herndon launched in a community centre nearly 25 years ago now attracts as many as 600 people to its seats in any given week.

"Nondenominational is actually the strongest force in American Christianity right now," said Ryan Burge, a professor who focuses on religion's impact on American life at Washington University in St. Louis. "They really talk about a personal relationship with Jesus Christ. Many of them preach a conservative gospel on things like abortion, same-sex marriage, but they don't lead with those things."

In 1972, fewer than 3% of Americans identified as nondenominational Christians. Now it's 14%, or nearly 40 million people, according to the General Social Survey.

Burge believes it's possible nondenominational Christians could overtake Roman Catholics in the next 15 years to be "the largest religious tradition in America."

"I think it's, we're moving away from authority structure," Burge said.

That looser structure is what attracts Rooftop members Anna and Nathan King, who grew up attending traditional Christian churches.

"Here at Rooftop, we focus on thinking critically about those traditions and challenging each other, but not letting it divide us," Anna said.

"I love how casual it is. It's really cool that, like, we could be in the pews and one of our elders or pastors is right in the pew next to us," Nathan said.

Herndon calls this "big tent Christianity." He uses videos and pop culture while focusing on what he sees as the fundamentals of the Bible, without taking a hard line.

When asked what he would say to people who view his church as "Christianity light," Herndon said, "I emphatically disagree. We dig really deep into scripture, we just try to figure out, what does this mean for people?"

He added, "Some denominations, more traditional denominations, I think they've realized, we don't have much of a future unless we try to figure out how to reconnect with people." (CBS News, but headline rejigged)

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