
For such a complicated phenomenon, there are almost certainly many causes. Still, I suspect that one is more important than the rest. If I had to pick one reason I suspect we are seeing a decline in religious affiliation, it would be the culture war being waged by fundamentalist Christian organizations such as the Catholic Church and the Southern Baptists. Essentially, they are giving religion a bad name in the eyes of the younger generation. By waging war against same-sex marriage, women, and a host of other issues, these groups have made themselves far less appealing.
I realize I may be engaging in some wishful thinking here. Perhaps I see this as the main reason for the decline because I want it to be the case. I just know that when I talk to young adults, I'm far more likely to hear support for same-sex marriage and confusion over why religious organizations continue to be on the wrong side of history than I remember from my youth.