Pew Research Center, this chart.  But do note, percent being Christian is declining.
http://www.pewforum.org/2015/05/12/americas-changing-religious-landscape/pr_15-05-12_rls-00/

The report itself "America’s Changing Religious Landscape" is actually very interesting, with  more in depth reading.
http://www.pewforum.org/2015/05/12/americas-changing-religious-landscape/