Self-professed atheists are the tip of a vastly larger secular population of nonbelievers and nominal believers.

In the American Religious Identification Survey (ARIS), with a sample size of more than 54,000, we can get representative data even on very small religious groups. Only 1.6% of Americans reported their religion as atheist or agnostic in 2008, but 15% (including the atheists and agnostics) reported having no religion. Another 5.2%, demographically similar to the no-religion people, said either that they do not know their religion or that they refused to answer the question.

Douglas Laycock, Secularization in the U.S.: Overblown or Underestimated?, Sightings (November 5, 2015).
UVA Law Faculty Affiliations