I don't know?
According to Wikepedia the closest definition is;
http://en.wikipedia.org/wiki/Antitheist
"Antitheism is defined by the Oxford English Dictionary as "One opposed to belief in the existence of a God.""
One definition has a Satanist as a antitheist. That would clearly be false for me. No God = no Devil.
It is wrong, what you said about Satanists, unless Satanists do not believe the way Satan himself does. It says right in the Bible that even the devil knows there is a God. He has to. Because God kicked Satan out of heaven.