Look up Census - Special Reports - "We the people" series
Since the 1960's no questions about religion have been included in the U.S. Census because of the constitutional separation of church and state. Here is an article from the Pew Forum on Religion & Public Life explaining how that developed.
The census webpage lists (but does not endorse) sources of religious statistics on this page: http://www.census.gov/prod/www/religion.htm
It also states that the Statistical Abstract of the United States has a few questions on religion in the population section. (see questions 75-77.)