The Pew Research Center surveyed over 35,000 American adults in 2014 as to their religious affiliation and compared it to a similar survey it had conducted in 2007. It’s key findings included:

• America is still a predominantly Christian nation with about 70% of the population identifying as some form of Christian.
• Overall, however, the percentage of American adults who identify as Christian is declining, having dropped from 78% to 70% in the seven year period between 2007 and 2014.
• The group with the largest increase in the survey was “unaffiliated” which grew by over 6%. Non-Christian religions such as Jewish, Muslim, Hindu and Buddhist also enjoyed small percentage increases.
• The largest declines occurred with Catholics and Mainline Protestant. Evangelical Protestants showed the smallest decline at about a 1% decline. Note that with the increase in population over the last seven years that the total number of Evangelicals actually increased slightly.
• The biggest driver of the increase of the religiously unaffiliated (or “nones”) is lack of “generational replacement” as the Millennials are reporting much lower connection with churches than older generations. Fewer than 60% of Millennials identify with any type of Christianity. The survey noted that generational replacement is not the only reason for the increase of “nones” – the religiously unaffiliated grew at all age levels over the seven year period.
• Geographically, the report found: “Religious “nones” now constitute 19% of the adult population in the South (up from 13% in 2007), 22% of the population in the Midwest (up from 16%), 25% of the population in the Northeast (up from 16%) and 28% of the population in the West (up from 21%). In the West, the religiously unaffiliated are more numerous than Catholics (23%), evangelicals (22%) and every other religious group.”

Link to Pew Research Report on Religious Affiliation: America’s Changing Religious Landscape