Gallup:

About 82% of Americans in 2007 told Gallup interviewers that they identified with a Christian religion. That includes 51% who said they were Protestant, 5% who were “other Christian,” 23% Roman Catholic, and 3% who named another Christian faith, including 2% Mormon. Because 11% said they had no religious identity at all, and another 2% didn’t answer, these results suggest that well more than 9 out of 10 Americans who identify with a religion are Christian in one way or the other.