How in the world could someone come to think of Satanism as atheistic?
Have you never read the Satanic Bible? Have you never heard of the Church of Satan or Anton LaVey?
"Satanists do not believe in the supernatural, in neither God nor the Devil. To the Satanist, he is his own God. Satan is a symbol of Man living as his prideful, carnal nature dictates. The reality behind Satan is simply the dark evolutionary force of entropy that permeates all of nature and provides the drive for survival and propagation inherent in all living things. Satan is not a conscious entity to be worshipped, rather a reservoir of power inside each human to be tapped at will. Thus any concept of sacrifice is rejected as a Christian aberration—in Satanism there’s no deity to which one can sacrifice."-Peter Gilmore, current leader of the Church of Satan.