But what of the other 50 percent? In the data, the only factor that correlates with this is date of birth—people born later are less likely to have a religious affiliation. But as Downey points out, year of birth cannot be a causal factor. “So about half of the observed change remains unexplained,” he says.
Increasing year of birth means increasing human knowledge. While it was convincing a long time ago to have a god for thunder, for fire, for love and a thousand other things, we learned that thunder works without any god. And now people learn that the universe works without any god at all, i.e. monotheistic religions are no longer as convincing as they used to be.