There is a difference Between atheist and Satanist- Please dont mix them up and then get into an argument about it even though they know what they are talking about.

Atheist : a person who does not believe in the existence of a god or any gods : one who subscribes to or advocates atheism.

Satanist: the worship of Satan or the powers of evil. a travesty of Christian rites in which Satan is worshiped. diabolical or satanic disposition, behavior, or activity.