Editor’s note: As the Christian landscape changes, leaders must ask and answer a new question: What’s the future of denominations? This reflection is part of an occasional series that offers insight on this vital issue. To see the entire series, click here.

The recent release of the National Council of Churches’ annual “Yearbook of American and Canadian Churches” adds fresh details to a familiar story: Membership is declining in the majority of Protestant denominations.

Compared with the previous year, Catholic membership is up 1.5 percent, but Protestant membership is down. For the second year in a row, the nation’s largest Protestant denomination -- the Southern Baptist Convention -- has reported a decline. And most black denominations are no longer growing.

The commonly cited reasons for this pattern are likewise familiar: Americans’ increasing secularization, postmodernism, competition from other activities, the aging of the baby boomers and churches’ lack of responsiveness to a changing culture.

No doubt all of these are part of the story. But there is another part of the story that gets overlooked: population decline. Although many people believe we’re in the midst of a worldwide population explosion, demographers and sociologists have known for quite some time that a couple of key factors actually lead to lowered birth rates: urbanization and the rising education and labor force participation of women.

In nations where those two factors exist, birth rates drop, precipitously. So much so, in fact, that soon those nations do not have enough children to replace themselves. And historically, nations whose fertility has dropped below the replacement level have not been able to bring it back above that threshold, no matter the policies instituted to do so.

In developed nations -- which have low infant death rates -- it takes 2.1 births per woman to exactly replace the population. Short of that, populations shrink, unless they are bolstered by immigration. Using the conservative 2.1 cutoff, about 70 nations are no longer having enough children to replace themselves, including the United States, with a fertility rate of 2.05, Iran (2.04), Ireland (1.96 ), Chile (1.94), Brazil (1.90), France (1.89), the UK (1.82), Australia (1.79), China (1.73), Canada (1.53), Cuba (1.49) and Japan (1.27).

Many others will join this group soon. In fact, using the low fertility assumption, United Nations figures indicate that in just 15 years the world as a whole will no longer replace its population. Given a lag effect, this means that in about 40 years, the world population will begin declining. This, in turn, means that relying on immigration to keep economies running will become increasingly difficult.

These demographic realities have vast implications for the church. My colleague George Yancey and I are only beginning to explore the implications, though some of them are becoming clear to us. Shrinking population will mean shrinking attendance figures. In the United States, population continues to grow for now because of immigrants, who also tend to have high birth rates. But native-born Americans of all races and ethnicities are not currently replacing themselves.

So denominations that largely draw their membership from native-born Americans have been declining and are likely to continue to decline in membership and attendance -- even absent the recognized factors. (Catholic membership, which has increased, includes a large number of immigrants.)

Sharing the gospel with those who have not yet heard it is a given, no matter what is happening in population trends. But Christian leaders still should begin thinking now about how to respond to the coming population decline.

Are we called to be fruitful and multiply -- and thus to have more children? Or do we take as a given that people will continue to have fewer children and that the church’s role is to aid in the adjustment?

If they take the former position, Christian leaders must prepare to go against the grain. And they must gear their focus -- teaching, theology and resources -- both to encouraging people to have more children and to engaging in social mobilization to reduce the barriers to having children.

If they take the latter position -- accepting population decline -- ministries will need to adapt to more than just smaller numbers in churches. There will be other changes as well. The population will consist of an ever-increasing number of older people. Economies worldwide will likely shrink (capitalism assumes growth), so perhaps the church will also need to respond by helping to find alternative economic models.

Much is yet unclear. What is clear is that we are in for seismic global change. We would do well to be ahead of the (declining) curve.