“An apology for the devil: it must be remembered that we have heard only one side of the case; God has written all the books.”
— Samuel Butler (1835-1902), English satirist. Dictionary of Humorous Quotations edited by Evan Esar
One would think that it would go without saying that atheism is not the same thing as satanism. Satan is merely another god. Atheists don't believe in any gods. Unfortunately, however, one often hears this nonsensical equation of atheism with satanism from believers.
Many times it comes from their complete ignorance. Having grown up surrounded by talk of nothing but god and his alleged opponent, satan, they think this simple bipolar world encompasses all possible scenarios. Therefore, anyone not working for god must be working for satan. They often also limit their notions of who is working for god to only those who represent their particular flavor of religion. Thus, to such people, even other Christians are often thought to be working for satan.
These people are an extreme example of the way in which religion necessarily leads to intolerance.
Others, however, have found a veneer of respectability for their bigotry in the words of non-believers. Many non-believers, aware of the sensibilities of believers, make a point of tweaking believers by calling their own views satanism or the like. Anton LeVay is a prime example. He called his philosophy of rational selfishness satanism even though there appears to be no reason to think that he actually believed in satan. It appears that he was simply stressing the point that he did not believe in god or religious morality.
One of the best responses to this nonsense is:
"You are more of a satanist than I am. You actually believe in satan."
"I don't believe in anything supernatural and that includes satan."
Others such as Milt Timmons, who wrote "Lucifer's Handbook", were simply trying to tweak the noses of the believers by pointing out, as did Samuel Butler in the quotation above, that it is far from certain who the "good guy" was in the mythical struggle between god and Lucifer. Judging from the Bible and the things believers say it appears that their god is psychologically similar to Kim Jong Il and his heaven is an eternal version of North Korea.
In that vein, another response might be:
"If Lucifer is the bad guy, then why does his name mean "bringer of light?"
(The actual answer to that question is that the name is a reference to a pagan god that was based on Venus--the morning star--whose appearance in the morning sky heralded the coming of the Sun. Most Christians won't know this, however. Thus, using this response is a slightly dishonest bit of nose tweaking, but some of them deserve it.)
In conclusion I would like to say that even though I understand the desire to have a bit of fun at believers' expense, it is not a good idea to associate non-belief with satan worship. Believers are already inclined to believe that we non-believers are evil and they have a history of persecuting us rather severely. Anything that lends any legitimacy to their tendencies toward doing such things hurts the cause of rationality and puts non-believers in danger.