When children raised by Christians grow up and leave the faith, there are societal consequences. There are still other consequences when these former Christians try to raise children of their own, and so on.
The generation whose parents were Christians drops the “supernatural” elements of their parents’ beliefs, thinking they’re unnecessary, but they still know right from wrong, they’re nice enough people, and they remember who Shadrach, Meshach, and Abednego were.
These nice people’s children drop their parents’ ethical code because it’s arbitrary and restrictive (mostly the latter). They have no idea who Moses was, or why he built that ark thing.
In turn, their children grow up thinking Santa Claus comes at Christmas because that’s when school lets out.
Then missionaries come from India to tell them about Jesus.
UPDATE 11 August 2011: “A hundred years ago most readers of The Picture of Dorian Gray, who were educated much as Wilde was, would have caught the reference; now even the experts are likely to miss it.” — Oscar Wilde and the Passion, by Alan Jacobs