Update (Mar. 15): A new report from the Institute for the Study of Societal Issues at the University of California, Berkeley, states that 1 in 5 U.S. adults reported no religious preference in the 2012 General Social Survey.
And although the percentage of respondents who specified a religious preference is declining, data suggests that belief in God actually is not declining. The report states:
[Respondents'] certainty of believing in God decreased more between 1965 and 1991 than since, while preference for no religion barely changed from 1965 to 1990, then almost tripled since 1991. This asymmetrical timing of changes indicates that the connection between faith in God and identifying with an organized faith, if there is one, is far from simple. Unchurched believers still far outnumbered completely secular people in 2012.
Update (Mar. 8): Bradley Wright has posted a new breakdown of the rise of the religiously unaffiliated over time based on age. Wright's newest analysis note that "the percentage of being unaffiliated increased in each group, but relatively speaking, it's increased most among the middle-aged and the elderly," rather than among those aged 18-29.
Update (Feb. 28): Bradley Wright has produced a chart noting how the biggest surge in "nones" actually occurred in the 1990s and has slowed since then.
According to a new Gallup report, the number of religiously unaffiliated Americans may not be surging as other research has suggested.
Gallup reports that the percentage of its survey respondents identifying as religiously unaffiliated rose an average of 1 percent per year between 2008 and 2011. Yet, such growth has slowed as "the 2011 to 2012 uptick in religious 'nones' is the smallest such year-to-year increase over the past five years of Gallup Daily tracking of religion in America."
This suggests that "religion may be maintaining itself or even increasing in the years ahead," Gallup editor-in-chief Frank Newport told CNN.
The Gallup results run counter to a widely cited report from the Pew Forum on Religion and Public Life. Released this past October, the Pew study found that nearly 20 percent of Americans identified as having "no religion"–and that the religiously unaffiliated are the fastest-growing "religious" group.