A demon never cares for any good instruction. He is just like a determined thief: one can give him moral instruction, but it will not be effective. Similarly, those who are demoniac or atheistic by nature can hardly assimilate any good instruction, however authorized it may be. That is the difference between demigods and demons. Those who can accept good instruction and try to live their lives in that way are called demigods, and those who are unable to take such good instruction are called demons.

From Srila Prabhupada's Krsna Book, Ch. 1: The Advent of Lord Krsna