I always wonder how many of the people who identify as a certain religion actually practice it. Most of the Catholics, Christians, and Jews I've met live their lives without consideration for the tenants of their faith.
I'd speculate that many of the so-called "Christian generic" group are nominal Christians at best. These are people who apparently weren't members of any organized religious denomination but who still told the interviewers that they were "Christians". (I can imagine them kind of shrugging their shoulders when they said it, with an implied "I guess...".)
Catholics are a notoriously unobservant lot. I know lots of people who identify themselves as Catholics because of ethnicity (as in "Irish Catholic") or upbringing (maybe they attended Catholic school) but who haven't attended mass in decades.
The Jews are interesting. Only 1.2% of Americans identified their religion as Jewish. But almost twice that percentage will say that they are Jewish if you ask them about their ethnicity. So close to half of self-identified American Jews no longer think of Judaism as their religion.
We probably also need to remember that religious
self-identification is something very different than religious
belief.
Here's some more data from ARIS (table 4 from page 8) where respondents were asked,
"Regarding the existence of God, do you think..."
There is no such thing -- 2.3%
There is no way to know -- 4.3%
I'm not sure -- 5.7%
There is a higher power but no personal God -- 12.1%
There is definitely a personal God -- 69.5%
Refused to answer -- 6.1%
So 2.3% are flat-out atheists, with another 10.0% agnostics, split between strong and weak varieties. Then there's another 12.1% that have some sort of transcedental intuitions but no conventional God-belief. That adds up to about 1/4 of the adult population. Just under 70% express belief in a personal God. (And I'd guess that for many of those, it's a pretty nominal belief, just the expected thing for them to say, and that it doesn't really influence their daily life very much at all.)
Interestingly, about 76% of Americans identify themselves as being generic "Christians" or as adherents of one of the many Christian denominations, so apparently almost 10% of self-identified American Christians don't even believe in a personal God. (Probably a lot of Episcopalians...) Clearly a certain amount of church adherence in the US is social, like membership in a club, or simply nominal, just what people tell interviewers.