Traditionally, denominations established new churches, or "planted" them, in areas of high population growth. Planting churches in existing communities was discouraged because it might lead to unseemly "sheep stealing"--attracting worshipers from other churches.

Yet even in growing denominations, congregations tend to plateau after 15 years, and membership starts to dwindle after 35 years, according to Mr. Stetzer's 2010 book, "Viral Churches," which he co-wrote with Mr. Bird. One way older churches overcome that decline is by starting "daughter" congregations, often in a nearby city using a core group of members from the "mother" church. Typically the daughter church maintains an affiliation with the mother church and its denomination.

Churches that launch a new congregation grow on average 22% in the five years after planting a church, while financial giving grows 48%, according to Mr. Stetzer's research. New churches also baptize new Christians at more than three times the rate of older churches

Much of the recent growth has been driven by individual churches deciding to start new congregations, rather than denominations directing the process. Manhattan's Redeemer Presbyterian Church has started 300 churches in 45 cities over the past 12 years, cooperating with 34 church-planting networks on five continents, according to its City to City network website. The church-planting group Acts 29 Network has started 500 churches over the past decade.

Yet denominations are still participating--even formal, hierarchical ones. The Anglican Church in North America hopes to launch 1,000 churches and has started more than 200 since 2009. Ethnic churches are among the fastest-growing. For example, the Nigerian denomination Redeemed Christian Church of God has grown from a single church in the U.S. in 1991 to about 400 in 2009.

All this helps explain why the physical building may be less important than it used to be. Church planting today relies on social networks, the families and especially the friends of existing members. Many church-planting networks create "missional communities," a loose term theologically, which simply means small, tightknit neighborhood groups. These groups gather to pray, share meals and help one another with child care, family difficulties or other daily struggles.

The approach seems to be an excellent way to win conversions. Sociologist Rodney Stark writes in "The Rise of Christianity" that the primary model for religious conversion is the spread of faith through social networks. According to his research, it takes Mormons 1,000 cold calls at residences to gain a convert, and the initial knock on the door is usually the first of many interactions. But "when missionaries make their first contact with a person in the home of a Mormon friend or relative of that person," he writes, "this results in conversion 50% of the time."

Person-to-person growth is one major pattern for growing religious movements, from Christian sects to newer faiths. New churches tend to have a higher percentage of converts, sometimes as much as 50%, according to Mr. Stetzer.

The expansion of congregations suggests that the drop in religious affiliation is not as dramatic as it seems, and that a stealthy revival might even be coming. The growth may not yet offset disaffiliation, but it is part of the American religious pattern. Early colonists included the highly religious Puritans. But their children and grandchildren strayed, even forcing churches to loosen qualifications if they were to keep members.

In the century that followed, Methodists and Baptists began spreading Christianity largely through small groups, or "bands," as the Methodists called them. They used nontraditional gathering places, including open fields, to bring their message to the masses. By 1850, 34% of Americans were church members, and by 1900 half were, according to Mr. Stark. By the early 1990s, nearly two-thirds of Americans were members of a congregation.

Fewer new churches these days are going up with drywall and spackling, but members are probably still stacking chairs and warming coffee on Sunday morning.

Rob Moll, author of "What Your Body Knows About God" (IVP Books, 2014), was a founding member of the vestry at Advent Anglican in Bellevue, Wash.