The relative shrinkage of the Christian percentage of the U.S. population in recent years means that other groups by definition have expanded. The Gallup trend data show that the most significant expansion has come among the group that does not identify with any religion, followed by a more modest expansion of those who identify with another, non-Christian, religion.
In 1948, 2% of Americans interviewed by Gallup volunteered that they had "no religion." The number stayed in that range until about 1970. By 1972, Gallup had measured 5% with "no religion." Gallup trends show the percentage gradually increasing since that time, with a very modest decline from an average of 8% in the early 1990s to 6% from 1993-1995, and then some fluctuation in the late 1990s, with the percentage settling in the 9% to 12% range since 2002. (Additionally, in 2008, an average of 3% of Americans did not answer the question on religious identity. Whether these people truly do not have a religious identity, or were confused or had several religious identities, is unknown.)
The group of Americans who are classified as "Other" includes the less than 1% who are estimated to be Muslim, and small percentages who are Buddhist, Hindu, or other non-Christian religions. The percentage of Americans classified in this Other category was near 0% in 1948, but was at 3% in 1949 and was generally in the 2% to 3% range through the 1950s and 1960s. It has risen in recent decades to as high as 8% in 1997, and is 7% today.
There are many theoretical explanations for the increase in those with no religious identity at the expense of those identifying with a Christian religion. Two social scientists at the National Opinion Research Corporation, Tom W. Smith and Seokho Kim, contemplating similar data from the General Social Survey in 2004, concluded: "In sum, an array of social forces from cohort turnover, to immigration, to reduced retention rates, indicate that the Protestant share of the population will continue to shrink and they will soon lose their majority position in American society."
One of the reasons I've dubbed this blog and this era The Breaking Time is because the old consensus', the old ideas that "everybody does A and B" are fracturing. We increasingly live in a world that has a dizzying array of ideals, possibilities and lifestyles. On that note, I doubt that Christianity is going to disappear, or that a form of Atheism (slippery a definition as that is) will predominate.
The idea of a national agreement on matters spiritual, let alone the numbers to back it up, is rapidly dying.