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.)
But, as Jon Meacham notes:
Let's be clear: while the percentage of Christians may be shrinking, rumors of the death of Christianity are greatly exaggerated. Being less Christian does not necessarily mean that America is post-Christian. A third of Americans say they are born again; this figure, along with the decline of politically moderate-to liberal mainline Protestants, led the ARIS authors to note that "these trends … suggest a movement towards more conservative beliefs and particularly to a more 'evangelical' outlook among Christians." With rising numbers of Hispanic immigrants bolstering the Roman Catholic Church in America, and given the popularity of Pentecostalism, a rapidly growing Christian milieu in the United States and globally, there is no doubt that the nation remains vibrantly religious—far more so, for instance, than Europe.
Meacham then proceeds to complicate this picture even further (as he always does), creating a rather complex portrait of the American religious landscape. See the whole thing here.