Barna does research on how "Post-Christian" America has become.
The data is really striking and I think shows a much more accurate picture of exactly how "Christian" the country actually is.
Being a former fundamentalist, one of the themes we had was that many people may claim to be Christian, but they aren't really. There were always claims of "people damned to hell may be sitting in these very pews", which was a call for people to come up and give themselves to Jesus.
Looking at life around me verified that things weren't completely legit in terms of following the Christian doctrines. Most people didn't know their bibles very well, they didn't read it often, or they had unsound doctrinal views compared to what the churches they were attending stated in their statements of faith/doctrine.
The main things I saw was that people would go to church, and then largely live like everyone else around them - like I live right now. Most people don't attend church regularly either but still report themselves as Christian, probably because they show up on XMas and Easter (the two weekends that your local church probably takes in the most money).
What's nice about this is that people in my generation and younger are significantly less religious, and it's likely to keep going that way. Which means when my daughter gets older, we hopefully won't have to even remotely worry about possibilities like President Rick Santorum, and the "Religious Right" won't have any significant political power anymore.