(RNS) Canadian researchers are revisiting a hotly debated sociological question: Why do some churches decline while others succeed?

Since the 1960s, overall membership in mainline Protestant Christian churches has been dropping in both the U.S. and Canada.

But some congregations have continued to grow, and a team of researchers believes it now knows why. It's the conservative theological beliefs of their members and clergy, according to researchers from Wilfrid Laurier University and Redeemer University College in Ontario.

"The riddle of mainline death has been solved," said David M. Haskell of Wilfrid Laurier University.

The results of the five-year research project will be published in the December issue of the Review of Religious Research. The project surveyed more than 2,200 churchgoers from Anglican, Presbyterian, Lutheran and United churches in Ontario, according to an executive summary. About half were part of 13 mainline Protestant congregations whose membership had declined at least 2 percent over the past decade, while the other half attended nine churches that had grown at the same rate. Researchers attended services at all 22 churches, after which they asked congregants to complete a 40-question survey, Haskell said. They also interviewed all clergy and a number of congregants who had completed the survey. In those interviews, most clergy and the congregants of the declining churches blamed changes in society for a drop in demand for religion. But, comparing the religious beliefs and practices of both the declining and growing churches, Haskell said more clergy and congregants of the growing churches held firmly to "traditional" Christian beliefs and were more diligent in practices such as prayer and Bible reading. That includes a more literal interpretation of the Bible and greater openness to the idea that God intervenes in the world, he said. For example, 93 percent of growing-church pastors said they agreed with the statement “Jesus rose from the dead with a real, flesh-and-blood body leaving behind an empty tomb,” compared with 56 percent of declining-church pastors, according to the summary. And 71 percent of pastors at growing churches said they read their Bibles daily, compared with 19 percent of pastors at declining churches.

Conservative Christians long have maintained their churches have continued to grow, even as membership in more progressive denominations has declined.

In 1972, researcher Dean M. Kelley published "Why Conservative Churches are Growing: A Study in Sociology of Religion." More recently, the trend has been noted by former Southern Baptist Convention President Albert Mohler, David Brooks of The New York Times and Joe Carter of The Gospel Coalition.

But even studies that agree conservative churches are growing still suggest there is no link between theology and that growth, Haskell said. Some have tied that growth to the age -- and birthrates -- of congregants or the age of the church itself.

"The strength of our study is we actually now can explain it: because theology matters," he said.