Before we get too cocky, one should recognize that although the fraction of the population that identifies as Christian is decreasing, it went from 78.4% to 70.6%. That is still a huge supermajority of the United States. The mechanism for the increase in the godless apparently is occurring from people primarily switching from being Catholic. The total number of Christians has declined from 178 million to 173 million.
Friendly Atheist notes that atheists and agnostics are a growing fraction of the unaffiliated:
Joe Jervis quotes from the report:
The decline of Christians in the U.S. has corresponded with the continued rise in the share of Americans with no religious affiliation (religious “nones”). People who self-identify as atheists or agnostics (about 7% of all U.S. adults), as well as those who say their religion is “nothing in particular,” now account for a combined 22.8% of U.S. adults – up from 16.1% in 2007. The growth of the “nones” has been powered in part by religious switching. Nearly one-in-five U.S. adults (18%) were raised as Christians or members of some other religion, but now say they have no religious affiliation.
Hat/tip to Joe.My.God and Friendly Atheist