We've seen that 1-in-5 Americans, and 1-in-3 among those under 30 years old, claim no religious affiliation. Similarly, we've seen that nearly half of all Americans (47% in 2014) and most of those under 30 (55%) do not attend worship services.
Lastly, let's take a look at how the percentages of those who are atheistic, agnostic, or vaguely theistic have changed over time. The data do not extend back as far for this item but most the secularizing shift has been more pronounced in the nineties and 2000s than it was in the seventies and eighties.
The atheistic, agnostic, and vaguely theistic categorizations are based on the following responses, respectively, to the command "Tell me which statement comes closest to expressing what you believe about God": "I don't believe in God", "I don't know whether there is a God and I don't believe there is any way to find out", and "I don't believe in a personal God, but I do believe in a higher power of some kind".
The following graph shows the percentages of respondents who are atheistic, agnostic, or vaguely theistic, for all respondents and for those under the age of 30, by year:
Not surprisingly, the results parallel quite closely those gauging a lack of religious affiliation over time, with 1-in-5 Americans (and 1-in-5 young adults) now believing in, at most, an undefined higher power.
GSS variables used: YEAR, AGE(18-29), GOD(1-3)(4-6)