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: yearbook.asarb@gmail.com 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 Though the Yearbook’s data are self-reported without auditing and sometimes out of date, it’s the best resource journalists and religious leaders have had for comparisons and as a source in which to quickly find numbers, contacts, and basics.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.  A new  January  estimate from Pew Research is 3.3 million, children included, as of 2015. Pew projects from present trends there will be 8.1 million by 2050. Pew’s prior counts were 2.75 million in 2011 and 2.35 million in 2007.  Pew uses “data on age, fertility, mortality, migration and religious switching drawn from multiple sources,” along with its own 2011 survey.

A competing current count for 2015 from the International Religious Demography Project is 4.4 million. Approximately 29 percent are African Americans. The project’s numbers are used by the Encyclopedia Brittanica Book of the Year and several annual almanacs.

This cooperate effort between a Boston University agency and Gordon-Conwell Theological Seminary’s Center for the Study of Global Christianity thinks 4.4 million most likely underestimates the real total, and projects 10 million U.S. Muslims by 2050. (Incidentally, the seminary center's third edition of the standard “World Christian Encyclopedia” is due in 2020.)

The project assesses Pew’s research but comes up with higher numbers. Asked why, Todd Johnson and Gina Zurio tell the Religion Guy they examine ethnic communities and immigrant populations through U.S. government sources and also scholars who specialize in specific  groups. They say most polls undercount immigrants’ religious affiliations because many groups are too small to identify when pollsters use the typical modest sample sizes. For example, the 90,000 Somali and 150,000 Bosnian immigrants count as overwhelmingly Muslim.    

Another significant number appeared in “The Americam Mosque 2011” from a team led by Ihsan Bagby at the University of Kentucky. This effort asked local mosque leaders to estimate attendance at their Eid festivals (rough equivalent of Christians’ Easter or Jews’ Yom Kippur) and reported 2.6 million “mosque participants.” Bagby figured that adding those who don’t attend mosques would bring an ethnic total of “up to 7 million,” the sort of debatable hunch Muslims frequently use.

With immigrant estimates, it’s important to note that many Americans with Arabic or Muslim-sounding names are actually Christians. For the Religion Guy’s further analysis of Muslim statistics, check out this previous post.