More Americans than ever attend non-denominational churches. Experts say it's a major shift in U.S. Christianity.

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

"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 non-denominational 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 center nearly 25 years ago now attracts as many as 600 people to its seats in any given week.

"Non-denominational 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. "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 non-denominational Christians. Now it's 14% or nearly 40 million people, according to the General Social Survey.

Burge believes it's possible non-denominational 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.