According to this study, the biggest „religious“ group (I prefer the more general term „Weltanschauung“) is the unaffiliated.
The authors (Robert P. Jones, Daniel Cox, Betsy Cooper, and Rachel Lienesch) write:
The American religious landscape has undergone substantial changes in recent years. However, one of the most consequential shifts in American religion has been the rise of religiously unaffiliated Americans.1 This trend emerged in the early 1990s. In 1991, only six percent of Americans identified their religious affiliation as “none,” and that number had not moved much since the early 1970s.2 By the end of the 1990s, 14% of the public claimed no religious affiliation. The rate of religious change accelerated further during the late 2000s and early 2010s, reaching 20% by 2012. Today, one-quarter (25%) of Americans claim no formal religious identity, making this group the single largest “religious group” in the U.S.
For the under-30-year olds the stats are even more striking. The change is momentous, the percentage has tripled over 30 years, the number of the unaffiliated „dwarf the percentages of other religious identifications“, the authors write:
Today, nearly four in ten (39%) young adults (ages 18-29) are religiously unaffiliated—three times the unaffiliated rate (13%) among seniors (ages 65 and older)…. In 1986, for example, only 10% of young adults claimed no religious affiliation.
Catholic (15%), white evangelical Protestant (9%), white mainline Protestant (8%), black Protestant (7%), other non-white Protestants3 (11%), and affiliation with a non-Christian religion (7%)
Of course, part of the argument lies in the method of breaking down religious affiliation quite finely into white Evangelical protestant, black protestant, mainline protestant etc. Taken together, of course the Christian organisations still dominate the religious landscape in the US. But the trends are clear, and they seem overwhelming in this study.
So maybe, when pundits point to the high levels of religious organization in the US, it is time to include a little caveat: The times, they are a-changing, the biggest belief group today is bigger than the Evangelicals or the Catholics, but the Unaffiliated.
Here’s the full paper