I just read, and really want to recommend, a post by David Madison on John Loftus' "Debunking Christianity" called The Christian Dark Ages -- Then and Now. Among the interesting insights here is the fact that during the 500 year period between the fall of the Roman empire and the end of the Dark Ages, a great deal of the knowledge accumulated by the Greeks and the Romans was literally erased and written over, and had to be re-learned hundreds of years later. The result of their following the advice to take no care for worldly things, because their Father in Heaven would take care of them (advice from Matthew 6 in the Sermon on the Mount), is that the world population plummeted: Millions of people starved to death. Villages disappeared and large cities became mere villages. The knowledge needed to sustain the population had simply been forgotten.
The "now" part of "then and now" is that belief in Satan and demon possession is on the rise. After more than 300 years of the age of Enlightenment, and 700 years of non-dark-ages preceding that, Americans especially are retreating into mythological beliefs in record numbers. Let's hope that the people who live in the real world continue to improve society's conditions despite the rejection of reason by so many.