Let's hold the above question for a moment and start with statistics about Christians in the United States.
Religion writers should be uttering hallelujahs for an organization many may not know about, the Association of Statisticians of American Religious Bodies. This association has just agreed to replace the National Council of Churches and rescue the invaluable “Yearbook of American and Canadian Churches.” This statistical compilation, issued since 1916, had been moribund since 2012 due to NCC financial woes. (Future contacts: firstname.lastname@example.org and www.asarb.org.)
The U.S. Census hasn’t asked about religious affiliations for decades, yet a writer often needs to report a denomination’s total adherents. Though the Yearbook’s data are self-reported without auditing and sometimes out of date, it’s the best resource journalists and religious leaders have had for comparisons and as a source in which to quickly find numbers, contacts, and basics.
The American Jewish Committee in 2009 likewise cut loose the 115-year-old “American Jewish Year Book,” taken over by the Springer book house. Jewish headcounts are complicated, but the 2014 annual estimated a population of 6.6 million to 6.7 million. The 2015 edition (list price $299!) has yet to appear. Meanwhile, the ubiquitous Pew Research Center figures the U.S. currently has 5.7 million “Jews by religion” as distinct from ethnic identity.
Moving to Islam’s U.S. followers, a number reporters would like to cite regularly, the following may not help much.