"Data included in the ARDA are submitted by the foremost religion scholars and research centers in the world. Currently housed in the Social Science Research Institute, the College of Liberal Arts, and the Department of Sociology at the Pennsylvania State University, the ARDA is funded by the Lilly Endowment, the John Templeton Foundation, Chapman University and the Pennsylvania State University.
From the US State Department's Office of International Religious Freedom, this page provides the Annual Report on International Religious Freedom 2016-2018. Older reports, going back to 2001, can be found by searching the archive.
U.S. Census Data
From the 2012 Statistical Abstract of the United States, tables on religious bodies and self-described religious identification.