If extrapolated to the national population, Barna's figures mean that more than 20 million adults in the U.S. attend a house church during the week, while 43 million do so once a month. And that does not even include the number of regular, traditional church goers who also participate in small group meetings.

It's a trend Barna thinks will accelerate. He believes that "by 2025 the local church will lose roughly half of its current 'market share' and that alternative forms of faith experience and expression will pick up the slack," home churches among them.

If the growing popularity of house churches is an indication that the institutional church model is not meeting the basic spiritual needs of Christians, perhaps some honest soul-searching on the part of church leaders is in order.

"Developing a biblical understanding of the preeminence of community life will take intentional leadership, strategic action and time," Barna said. In considering how to meet such needs, Barna's recommendations on another subject -- getting the unchurched back into church -- are relevant.

"These people tend to be less turned on by the music or preaching than by a sense of God's presence -- even though they don't quite know how to explain or understand it -- and by the feeling that they are visiting a group of people who are a genuine community of loving and accepting individuals," Barna said about the unchurched.

It may not be clear just yet if there is a genuine shift occurring in the religious life of America. But what is clear is that, if institutional churches want to remain relevant, they can no longer conduct business as usual.