An interesting read from the LA Times.
A couple of quotes:
When a church doesn't take itself seriously, neither do its members. It is hard to believe that as recently as 1960, members of mainline churches — Episcopalians, Presbyterians, Methodists, Lutherans and the like — accounted for 40% of all American Protestants. Today, it's more like 12% (17 million out of 135 million). Some of the precipitous decline is due to lower birthrates among the generally blue-state mainliners, but it also is clear that millions of mainline adherents (and especially their children) have simply walked out of the pews never to return.
When your religion says "whatever" on doctrinal matters, regards Jesus as just another wise teacher, refuses on principle to evangelize and lets you do pretty much what you want, it's a short step to deciding that one of the things you don't want to do is get up on Sunday morning and go to church.