John Green, a senior research fellow and a principal author, attributed this to the decline in birthrates, the inability to retain people born into the churches and people raised mainline Protestant moving to the ranks of the unaffiliated. "You might sort of think of this as family problems -- both at the level of having children, and raising children," Green said.
"Everybody in this country is losing members; everybody is gaining members," said Luis E. Lugo, director of Pew Forum on Religion & Public Life.
"It is a very competitive marketplace and if you rest on your laurels, you're going to be history."
Despite the decline of institutional religion, Mouw said, "we should not jump too quickly to the conclusion that secularism has taken over."
"There is much spirituality out there among the unaffiliated. I find this to be an exciting challenge."
For example, many parents may be content with their own "traditional patterns of worship," he said, but also know that their children are turned off by them.
"So they look for something that the whole family can commit to. Is that treating religion as a mere 'commodity'? Hardly. It is struggling to find resources that will help us deal with some of the most profound and intimate issues of our lives."
To view the study, visit www.pewforum.org.