In the Bhagavad-gītā (16.7-20) the Supreme Lord has exhaustively described the nature of such atheistic demons:
Those who are demoniac do not know what is to be done and what is not to be done. Neither cleanliness nor proper behaviour nor truth is found in them. They say that this world is unreal, with no foundation, no God in control. They say it is produced of sex desire and has no cause other than lust. Following such conclusions, the demoniac, who are lost to themselves and who have no intelligence, engage in unbeneficial, horrible works meant to destroy the world. Taking shelter of insatiable lust and absorbed in the conceit of pride and false prestige, the demoniac, thus illusioned, are always sworn to unclean work, attracted by the impermanent.