The survey is conducted by researchers at Trinity College in Hartford, Conn., and funded by the Lilly Endowment and the Posen Foundation. Conducted in 1990, 2001 and last year, it is one of the nation's largest major surveys of religion.
Meanwhile, the number of people who use nondenominational terms has gone from 194,000 in 1990 to more than 8 million.
"There is now this shift in the non-Catholic population -- and maybe among American Christians in general -- into a sort of generic, soft evangelicalism," said Mark Silk, who directs Trinity's Program on Public Values and helped supervise the survey.
The survey substantiated several general trends already identified by sociologists: the slipping importance of denomination in America, the growing number of people who say they have "no" religion and the increase in religious minorities including Muslims, Mormons and such movements as Wicca and paganism.
The only group that grew in every U.S. state since the 2001 survey was people saying they had "no" religion; the survey says this group is now 15 percent of the population. Silk said this group is likely responsible for the shrinking percentage of Christians in the United States.
Northern New England has surpassed the Pacific Northwest as the least religious section of the country; 34 percent of Vermont residents say they have "no religion." The report said that the country has a "growing non-religious or irreligious minority." Twenty-seven percent of those interviewed said they did not expect to have a religious funeral or service when they died, and 30 percent of people who had married said their service was not religious. Those questions weren't asked in previous surveys.
The survey reflects a key question that demographers, sociologists and political scientists have been asking in recent years: Who makes up this growing group of evangelicals? Forty-four percent of America's 77 million Christian adults say they are born again or evangelical. Meanwhile, 18 percent of Catholics also chose that label, as did 40 percent of mainline Christians.
"If people call themselves 'evangelical,' it doesn't tell you as much as you think it tells you about what kind of church they go to," Silk said. "It deepens the conundrum about who evangelicals are."