You often hear this little Orwellian tidbit from those who are in denial regarding the fact that the Dark Ages were caused by Christianity. (Some are in denial simply because it hurts their egos to think they are part of such an evil group; others are in denial for the additional reason that they want to drag humanity back into that cesspool--only with better weapons this time.) If you hear it, you can reply with:
"The church burned entire libraries and preserved only those books it found suitable because they weren't a threat to the church. The church wasn't "preserving" knowledge during the dark ages, it was controlling it."
This is just another Big Lie, another attempt to poison the well in the marketplace of ideas, another example of the religious re-writing history to make themselves look good. An unknown number of ancient works were destroyed by the Christians. The Dark Ages ended and the Renaissance began only because ancient works were discovered preserved in libraries of the Muslims, who, though hardly more tolerant these days, showed a great deal of tolerance during the European Dark Ages.
The truth about the church's motives and actions in this regard is instructive regarding its current actions. The church wants very much to once again have control over knowledge so that it can prevent apostasy using ignorance and manipulation. This insight goes a long way toward explaining their insane insistence on organized school prayer.
That issue is their new "wedge strategy" to try to force the enactment of school voucher laws or, better yet, scrap the public school system altogether. The result of such a change, of course, would be to turn over almost all education to churches. If you don't think this is important to the churches, take a look at recent history in countries like Spain and Chile where governments tried to do the opposite and kick religion out of the education business. The churches literally overthrew the elected governments in those countries because of it.