The overall religious identification of the United States adult population.
Harvard Pluralism Project - http://www.pluralism.org/index.php
In the past forty years, immigration has dramatically changed the religious landscape of the United States. Today, the encounter between people of different religious traditions takes place in our own cities and neighborhoods. In 1991, the Pluralism Project at Harvard University began a pioneering study of America's changing religious landscape.
U.S. Department of Justice: Religious Freedom - http://www.usdoj.gov/crt/religdisc/religdisc.html
Religious liberty is often referred to as the "First Freedom" because the Framers placed it first in the Bill of Rights. Yet it is not merely first in order: it is a fundamental freedom on which so many of our other freedoms rest.