I have a PDF of the 2008 ARIS survey and can quote some figures from it.
Here are 2008 religious identification figures. I underlined 'identification', since as Fraggle notes, that isn't the same thing as belief. There are religious believers who aren't associated with any church, while many people who don't have belief still claim some religious affiliation, typically for ethnic reasons. Religious identification isn't the same thing as religious participation either. Many people who identify with a church never actually show up there except for funerals and weddings.
When American adults were asked in 2008 what religious group they identified with, here's how ARIS classified the responses:
Catholic 57,199,000 25.1%
Baptist 36,148,000 15.8%
Nones/No Religion 34,169,000 15.0%
'Christian' Generic 32,441,000 14.2%
'Mainline' Christian 29,375,000 12.9%
Dont Know/Refused 11,815,000 5.2%
Pentecostal/Charismatic 7,948,000 3.5%
Protestant 'Denominations' 7,131,000 3.1%
Mormon/LDS 3,158,000 1.4%
New Religious Movements and Other 2,804,000 1.2%
Jewish 2,680,000 1.2%
Eastern Religions 1,961,000 0.9%
Muslim 1,349,000 0.6%
My comments:
The Catholic percentage of the American population is rising, largely because of immigration (legal or otherwise) from Latin America. Many of the European-descended Catholics (Irish and Italian, etc.) are only nominally Catholic.
The Baptists, just by their nature (emphasizing adult baptism), are the prototypical 'born agains'.
The 'Christian' Generic category are those people who told the pollsters that they were 'Christians', but didn't state a particular organized church. Some of these people may be fundies who belong to independent store-front congregations. But my feeling is that the bulk of them are nominal Christians who never attend church, can't even think of a group that they identify with, but still vaguely identify with the Christian tradition in general.
'Mainline Christian' refers to the Episcopalians, Presbyterians and Lutherans, along with the Methodists. This group generally are the northern European state churches and in the US they represent the old-stock English, Scottish, German and Scandinavian settlers. This group is typically more upscale and better educated than the average American. These 'mainline' groups produce a great deal of the more liberal theological scholarship (Biblical criticism etc.) and much of their membership are unconventional or nominal believers who attend for social reasons. As a result, these groups are slowly shrinking as they experience attrition at both ends, with many nominal members drifting over to the 'none's' while the more theologically conservative believers defect to the more Biblical Baptists.
The Protestant 'Denominations' are the Churches of Christ, Jehovah's Witnesses and Seventh Day Adventists, along with many smaller denominations of the same sort.
The Mormon/LDS category is holding steady at about 1.4% of the US population. Mormons have larger families than average, but this is a church that originated here in America and isn't being strengthened by immigration.
The number of people volunteering 'Jewish' is twice as large (more than 5 million) if poll-takers ask about ethnicity rather than religious identification. Which implies that perhaps half of America's self-identified ethnic Jews don't think of Judaism as their religion.
The 5.2% who refused to answer or said 'don't know' represent a large group. It's interesting to speculate what these people are really thinking. Many of those who refused to paticipate may have conventional affiliations but just didn't want to be bothered by the poll. But 'don't know'? That suggests a pretty low level of religious identification to me. So many of these people probably belong among the 'nones'.
The 'New Religious Movements and Other category' is surprisingly big too, at 2.8 million people. This one includes the many varieties of 'New Age' spirituality, 'Wicca' and similar things. It's very visible here in California.
The largest component of Eastern Religions are the Buddhists, at 1,189,000 0.5%. Interestingly, the Eastern Religions group is the best-educated in the United States, at 59% college graduates.