According to the American Religious Identification Survey that got Mohler's attention, the percentage of self-identified Christians has fallen 10 percentage points since 1990, from 86 to 76 percent. The Jewish population is 1.2 percent; the Muslim, 0.6 percent. A separate Pew Forum poll echoed the ARIS finding, reporting that the percentage of people who say they are unaffiliated with any particular faith has doubled in recent years, to 16 percent; in terms of voting, this group grew from 5 percent in 1988 to 12 percent in 2008—roughly the same percentage of the electorate as African-Americans. (Seventy-five percent of unaffiliated voters chose Barack Obama, a Christian.) Meanwhile, the number of people willing to describe themselves as atheist or agnostic has increased about fourfold from 1990 to 2009, from 1 million to about 3.6 million. (That is about double the number of, say, Episcopalians in the United States.)
I love the last line about Episcopalians, a denomination that is shrinking faster than new wool in hot water. Why? Because they, like many mainline Protestant groups, have ceased to be Christian.