Excerpt:
In the debate over a plan to build an Islamic center near Ground Zero in New York, one number has been elusive: the population of Muslims in the city. Commentators have cited figures that have been all over the map—from 600,000 in the city's five boroughs to between 600,000 and 800,000 in Manhattan alone.
That is because no one knows for sure how many Muslims, or Jews, or Christians, live in New York or anywhere else in the U.S. The Census Bureau doesn't ask Americans to disclose their faith, and the surveys and studies that attempt to fill in the gap come up with different numbers—particularly when it comes to relatively small religious groups. That leaves the U.S. behind much of the world when it comes to understanding the religious beliefs of its inhabitants, and has sparked controversy about the number of Muslims in the country.
These are important numbers because "religion is very consequential in how people see the world, view politicians, and view social, economic and even environmental issues," says Barry Kosmin, a sociologist at Trinity College and a principal investigator with the American Religious Identification Survey, which has surveyed Americans about their faith three times in the past decade.