Gallup classifies Americans into three religious groups based on their responses to a question measuring religious service attendance and how important religion is in their daily life. Very religious Americans are those who say religion is important to them and who attend services every week or almost every week. Nonreligious Americans are those for whom religion is not important and who seldom or never attend religious services. Moderately religious Americans meet just one of the criteria, either saying religion is important or that they attend services almost every week or more.

Gallup began tracking several religious indicators on a daily basis in 2008. Some of these indicators have shown significant change over this time, most notably the percentage of Americans who report no formal religious identity when asked to name their religious preference. But the percentage classified as very religious on the basis of their attendance and view on the importance of religion has stayed remarkably stable. In 2008, 41% of Americans were very religious, 29% moderately religious and 30% nonreligious. In 2015, those same percentages are almost identical: 40%, 29% and 31%, respectively.