Mainline Protestant churches are in trouble: a 2015 report by the Pew Research Centre found that these congregations, once a mainstay of American religion, are now shrinking by about one million members annually. Fewer members not only means fewer souls saved, a frightening thought for some clergy members, but also less income for churches, further ensuring their decline.

Faced with this troubling development, clergy members have made various efforts to revive church attendance. It was almost 20 years ago that the Right Reverend John Shelby Spong, an American bishop in the Episcopalian Church, published his book Why Christianity Must Change or Die. It was presented as an antidote to the crisis of decline in mainline churches. Bishop Spong, a theological liberal, said congregations would grow if they abandoned their literal interpretation of the Bible and transformed along with changing times.

Bishop Spong’s general thesis is popular with many mainline Protestants, including those in the United Methodist, Evangelical Lutheran, Presbyterian and Episcopal churches. Bishop Spong’s work has won favour with academics, too. Praising the Bishop’s work specifically, Karen L. King, of Harvard Divinity School, said in a review of his book that it “should be required reading for everyone concerned with facing head-on the intellectual and spiritual challenges of late 20th-century religious life”.

Harvard Divinity professor and liberal theologian Harvey Cox said “Bishop Spong’s work is a significant accomplishment”, and indeed, Cox himself has long been at the task of shifting Christianity to meet the needs of the modern world. Thus, liberal theology has been taught for decades in mainline seminaries and preached from many mainline pulpits. Its enduring appeal to embattled clergy members is that it gives intellectual respectability to religious ideas that, on the surface, may appear far-fetched to modern audiences.

But the liberal turn in mainline churches does not appear to have solved their problem of decline.

Over the past five years, my colleagues and I conducted a study of 22 mainline congregations in the province of Ontario. We compared those in the sample that were growing mainline congregations to those that were declining. After statistically analysing the survey responses of more than 2,200 congregants and the clergy members who serve them, we came to a counterintuitive discovery: Conservative Protestant theology, with its more literal view of the Bible, is a significant predictor of church growth while liberal theology leads to decline. The results were published this month in the peer-reviewed journal Review of Religious Research.

We also found that for all measures, growing church clergy members were most conservative theologically, followed by their congregants, who were themselves followed by the congregants of the declining churches and then the declining church clergy members. In other words, growing church clergy members are the most theologically conservative, while declining church clergy members are the least. Their congregations meet more in the middle.

For example, we found that 93 per cent of clergy members and 83 per cent of worshippers from growing churches agreed with the statement “Jesus rose from the dead with a real flesh-and-blood body, leaving behind an empty tomb”.

This compared with 67 per cent of worshippers and 56 per cent of clergy members from declining churches. Furthermore, all growing church clergy members and 90 per cent of their worshippers agreed that “God performs miracles in answer to prayers”, compared with 80 per cent of worshippers and a mere 44 per cent of clergy members from declining churches.

Outside our research, when growing churches have been identified by other studies — nationally and internationally — they have been almost exclusively conservative in doctrine. As we explain in our academic work, because of methodological limitations, these other studies did not link growth to theology. But our work suggests this is a fruitful avenue of research to pursue.

What explains the growth gap between liberal and conservative congregations? In defence of liberal churches, one may venture that it is the strength of belief, not the specifics of belief, that is the real cause of growth. In this case, pastors embracing liberal theology are just as likely as conservative pastors to experience church growth, provided they are firm and clear in their religious convictions. Yet different beliefs, although equally strong, produce different outcomes.

For example, because of their conservative outlook, the growing church clergy members in our study took Jesus’s command to “Go make disciples” literally. Thus, they all held the conviction that it is “very important to encourage non-Christians to become Christians”, and thus likely put effort into converting non-Christians.

Conversely, because of their liberal leanings, half the clergy members at the declining churches held the opposite conviction, believing that it is not desirable to convert non-Christians. Some of them felt, for instance, that peddling their religion outside of their immediate faith community is culturally insensitive.

It should be obvious which of these two convictions is more likely to generate church growth.

While our research helps to explain the dwindling ranks of liberal mainline congregations, it is not likely to bring much “joy to the world” of mainliners, especially those on the theological Left. But, if it is any consolation, when it comes to growth in mainline churches, Bishop Spong and other liberals are right to claim that Christianity must change or die.

They just get the direction of the change wrong.

•David Millard Haskell is a professor of religion and culture at Wilfrid Laurier University