Those who don't attend church regularly but are scouting for Christmas services this year might be surprised to find fewer mainline Protestant churches around these days.

Across the English-speaking world the numerical decline of mainline Protestantism is accelerating. The largest mainline Protestant denominations in the United States are the United Methodist Church, the Evangelical Lutheran Church in America, the Presbyterian Church (U.S.A.) and the Episcopal Church. Collectively, membership in these denominations decreases by about 1 million a year, resulting in hundreds of church closures annually.

While most mainline Protestant churches are declining, there's been no consensus as to why. Hoping to solve this sociological riddle, some colleagues and I conducted a study. We tracked down an elusive sample of growing mainline congregations and compared them to a sample of declining congregations. We surveyed more than 2,200 of the congregants, half attending growing churches and half at declining churches, and the clergy who serve them.

We found, without exception, the clergy and congregants of the growing mainline Protestant churches held more firmly to traditional Christian beliefs, such as the belief Jesus rose physically from the grave and that God answers prayer. The clergy of the growing churches were the most theologically conservative and the declining church clergy the least.

When we used statistical analysis to determine which factors are influencing growth, conservative Protestant theology, with its more literal take on the Bible, was a significant predictor. Conversely, the analysis showed liberal theology, with its metaphorical reading of Scripture, leads to decline. Our research stands out because past studies have suggested theology and church growth are not linked. They are.

As you might imagine, our results have been well-received by theological conservatives. Liberal Christians have been a lot less satisfied.

Some tell us that liberal mainline Protestant churches do grow and they know this because they've seen it. But this is anecdotal fallacy. A single unverified case can't be used to contradict strong statistical evidence.

In terms of strong statistical evidence, we point out our study is not alone in concluding that churches with conservative doctrine grow. For example, "The Faith Communities Today Study" analyzed data from thousands of congregations across the United States. That study found, by far, growing churches had clergy and congregants who were theological conservatives. Unlike our research, that study made no link between theology and growth, but as we point out in our academic work, it didn't fully explore the issue. It asked just one question of one person, the church pastor, to gauge theological outlook of an entire congregation. We surveyed all the congregants and the clergy in each church and asked many theological questions. Better questions elicit more accurate results.

It's also commonly asserted by our liberal critics that it is not the type of theology that matters for church growth but whether the theology is believed strongly and articulated clearly. However, we would suggest that different convictions, though equally strong and clear, produce different outcomes.

For example, all the growing church clergy in our study, because of their theological outlook, held the conviction that it is "very important to encourage non-Christians to become Christians." As theological conservatives, these pastors believe Jesus is the only way to salvation and that they must "Go and make disciples everywhere."

Conversely, half the clergy at the declining churches held the opposite conviction, believing it is not desirable to convert non-Christians. As theological liberals, these pastors believe there are many paths to salvation and that it's culturally insensitive to peddle your beliefs on those outside your religious community. Comparing the two theological outlooks, which do you think is more likely to generate church growth?

As social scientists, my colleagues and I are not advocating the theological rightness of one doctrinal position over another. But, if we are talking solely about church growth, conservative Protestant doctrine is the clear winner. With a nod to the season, mainline clergy and congregants with conservative outlook are more apt to be singing "Silent Night" this Christmas and in future years; theological liberals risk a different kind of silent night.

David Millard Haskell is associate professor of religion, culture, and digital media and journalism at Wilfrid Laurier University, Brantford Campus, Ontario, Canada. He wrote this for InsideSources.com.