Chapter 20, "City of God" by Saint Augustine

Of the Kind of Knowledge Which Puffs Up the Demons 

However, the very origin of the name suggests something worthy of consideration, if we compare it with the divine books. They are called demons from a Greek word meaning knowledge. Now the apostle, speaking with the Holy Spirit, says, Knowledge puffs up, but charity builds up. 1 Corinthians 8:1 And this can only be understood as meaning that without charity knowledge does no good, but inflates a man or magnifies him with an empty windiness. The demons, then, have knowledge without charity, and are thereby so inflated or proud, that they crave those divine honors and religious services which they know to be due to the true God, and still, as far as they can, exact these from all over whom they have influence. Against this pride of the demons, under which the human race was held subject as its merited punishment, there was exerted the mighty influence of the humility of God, who appeared in the form of a servant; but men, resembling the demons in pride, but not in knowledge, and being puffed up with uncleanness, failed to recognize Him.

http://www.newadvent.org/fathers/120109.htm