On the Pew U.S. Religious Landscape Survey, 1
The Pew Forum on Religion and Public Life has released a new survey on the landscape of American religion that will be the benchmark study of this subject for a decade. It is big enough, at 36,000+ respondents, and diverse enough to give a picture of denominational groups down to .3 of the population.
I want to start with the big lumps of denominational families that it reveals. Americans are
1/4th Evangelical Protestant
1/4th Mainline + Black Protestant
1/4th Everything else
Of the Everything else, about half are "nothing in particular" - but half of them say that religion is important in their lives.
All the non-Christian religions add up to about 5% of the total U.S. population.