You might be interested in a new Barna study that shows that the percentage of US citizens that are religiously unaffiliated is 20%. https://www.barna.com/research/state-church-2016/#.V-NCwigrI2w. I bring it up because Pew Research showed in 2014 that the unaffiliated was 23%. Of course different polling agencies have different methods and have different results. The Barna poll is based on 5,000+ interviews and also shows that 90% of respondents believed in God.
The Barna study is only marginally relevant as most "Nones"/unaffiliated are theists and not atheists. See: http://www.conservapedia.com/Nones