Pew Research Center built its religion research program on a method it uses across all its polling: large probability based surveys asked the same way across years so change can be measured rather than guessed at. The Religious Landscape Study is the flagship, a survey of more than thirty five thousand American adults fielded in 2007, 2014 and again in 2023 to 2024, each wave sorting respondents into named Christian traditions, Evangelical, Mainline and Historically Black Protestant, Catholic, Orthodox and several smaller groups, named non Christian traditions and three separate categories of religious unaffiliation, atheist, agnostic and nothing in particular, rather than lumping every unaffiliated respondent into one row.
The Center's global work extends the same method outward. Its 2012 Global Religious Landscape report and its 2025 study of how the picture changed between 2010 and 2020 estimate the religious composition of more than two hundred countries and territories, the figures this atlas draws on for national and state level religious affiliation facts wherever a country or an American state carries one.
Because Pew is organized specifically as a nonpartisan fact tank that takes no policy position of its own, its religion numbers are cited here as a demographic snapshot rather than as an argument for any conclusion about belief. Where a country's religious composition is disputed or a state's affiliation figures shift between survey waves, the atlas records what the Center's own methodology section says about margin of error and survey design rather than treating either wave's number as final.