“Sunday morning is the most segregated hour of Christian America.”

That declaration, which has been attributed to Rev. Martin Luther King Jr., used to startle listeners. Now it’s virtually become a cliché. For years, various academic studies and news articles have reported what many churchgoers already know: most American congregations are segregated.

In the latest issue of the academic journal Sociological Inquiry, two professors dug deeper into why Sundays remain so segregated.

The article, “Race, Diversity, and Membership Duration in Religious Congregations,’ said that nine out of ten congregations in the U.S. are segregated - a single racial groups accounts for more than 80 percent of their membership.

Kevin Dougherty , a sociology professor at Baylor University in Texas, and a co-author of the article, says churches haven't kept pace with other institutions.

Socially, we’ve become much more integrated in schools, the military and businesses. But in the places where we worship, segregation still seems to be the norm.

Attracting members of another race isn’t good enough, Dougherty says. Most congregations that draw racial minorities can’t keep them.

It’s not just an issue of attraction, of getting them into the door, but of retention. Can we keep them? Our research indicates that we’ve not been able to.

Calling a church segregated may make some people uncomfortable because it implies that its members are racist. But many contemporary churches that are dominated by one racial group weren’t formed by racial animosity, Dougherty says.

Parishioners’ prefer to go to church with people who look like them, Dougherty says.

People choose churches where they feel comfortable. Maybe they get challenges there, but they’re going for the comfort.”

The first Christian church was known for its diversity. Jews, Gentiles, and Greeks mingled alongside women and slaves. Biblical scholars have long maintained that the early church’s diversity was one of the reasons it became so popular. Roman society was characterized by rigid ethnic and class divisions.

That was then, though.

How important is it for the church to strive for that kind of ethnic diversity today - particularly since it seems many people don’t want it, according to the article?

Can American churches remain separate but equal?

Editor's Note: CNN's Soledad O'Brien looks at how some are fighting debt from the pulpit in "Almighty Debt: A Black in America Special," premiering October 21 at 8 p.m. ET.