U.S. Religious Landscape Survey

You can find details on the religious composition of the United States, including religious makeup, religious beliefs and practices in the U.S. Religious Landscape Survey. More than a study of religion, the Pew Research survey also includes the social and political attitudes affiliated with religious traditions in the United States. The survey is based on interviews with more than 35,000 Americans.

One element I found particularly interesting is A Brief History of Religion and the U.S. Census. From the document:

The U.S. Census Bureau has not asked questions about religion since the 1950s, but the federal government did gather some information about religion for about a century before that. Starting in 1850, census takers began asking a few questions about religious organizations as part of the decennial census that collected demographic and social statistics from the general population as well as economic data from business establishments...Although the census takers did not interview individual worshipers or ask about the religious affiliations of the general population, they did ask members of the clergy to identify their denomination – such as Methodist, Roman Catholic or Old School Presbyterian. The 1850 census found that here were 18 principal denominations in the U.S.

The same basic questions on religious institutions were included in the 1860 and 1870 censuses. In 1880, census takers started collecting more in-depth information from religious leaders on topics ranging from average worship attendance to church income, expenditures and debt. The scope of inquiry about religion was expanded again in 1890, when census takers gathered information about the number of ministers in each denomination. Classifications for the denominations also were more detailed...

There were no other significant changes in data collection on religious bodies until 1902, when the U.S. Census Bureau was established as a permanent government agency and census officials decided to separate some data collection from the regular decennial census. This led to the statutory creation of the Census of Religious Bodies, which began in 1906 as a stand-alone census to be taken every 10 years.