Back in the 1930s and 1940s, the number of "nones" -- those who said they were religiously unaffiliated -- hovered around 5 percent, Claude Fischer, one of the researchers with UC Berkeley, told The Huffington Post. That number had risen to only 8 percent by 1990.
But since then, the number of people who don't consider themselves part of a religion has increased to 20 percent.
MORE: http://www.huffingtonpost.com/2013/03/13/religion-america-decline-low-no-affiliation-report_n_2867626.html
See also...
Americans and Religion Increasingly Parting Ways
http://www.sciencedaily.com/releases/2013/03/130313100438.htm
Yet Another Survey Shows ‘Nones’ Growth at Record Levels
http://www.religiondispatches.org/dispatches/elizabethdrescher/6925/yet_another_survey_shows__nones__growth_at_record_levels/
