@WhiteLion:
I think you deserve +rep for your knowledge on the dark ages.
I've looked around for theories for Dark Ages and found a very interesting and plausible answer to it. The eruption of Krakatau.
If this volcanic hypothesis is correct, the global, domino-like effects upon epidemics, agriculture, politics, economics, and religion are far-reaching, elevating the potential role of volcanism as a major climate control, and demonstrating the intimate link between human affairs and nature.
(Additional information about the eruption of Krakatau and its causes
here)
As it said, the result of the volcano could have caused a catastrophic chain reaction that caused everything to fall apart (the economy being the main subject here). Even if this theory doesn't seem plausible to you (Though with the evidence is staggering in the link provided), the fall of Rome took a giant toll on everyone, and the fact that serfdom took over meant that everyone was too busy working for survival to actually worry about education.
So in reality what WhiteLion said is true, we actually owe Christianity the knowledge they preserved.
You also can't deny the fact that because of Christianity, the printing press was invented and one of the most widely spread piece of work was the bible in which many learned to read from.