You probably know this, but I only heard it the other day and it stunned me. Roman Catholics form the largest religious denomination in the United States.
The second biggest group: Former Catholics.
Update: Michael, a top notch Communications scholar and dear friend, challenges the truth of my post this morning. He writes:
I think these figures are incorrect. The Pew survey of 2008 shows 23.9% of US adults regard themselves as Catholics, differentiated from the 31.4% who were raised Catholic. It would be a stretch to get the number of former Catholics to more than 47% as would be required in your report were true.