It's not just Christians who don't believe in God any more - check out Rabbi Jeffrey Falick's intriguing blog: http://www.theatheistrabbi.com
The "orientation model" of religion appears to be growing; fewer and fewer people adhere to the "belief model" (OK, they are still a majority). Perhaps this is due to better education. Perhaps it is a consequence of people being exposed to other viewpoints, and realising that what they previously held as "truth" is really just a societal custom, open to change.