Further to what SBF says, quoting from page 3 of the Pew Report of 12 May 2015:
[T]he percentage of adults (ages 18 and older) who describe themselves as Christians has dropped by nearly eight percentage points in just seven years, from 78.4% in... 2007 to 70.6% in 2014.
So in 7 years, the number of Christians in USA dropped by 10%. It is such a major change, that I am surprised more people haven't mentioned it. If that rate were to continue, USA will have a majority of unbelievers in around 20 years.