Just a tiny technical detail:  Satanism is not an argument against theism, it *is* theism.  What's cuter still is the common variants are little more than Christian striation cults.  From simplistic inversions of Christian teachings and symbols (Good->Evil, Cross->Upside-down Cross) to sublime interpretations of Sufis*, who worship Satan as the greatest lover of God -- like Christ, receiving the ultimate punishment for embodying the ultimate truth**.

*Islam, at least for the nonbelievers: Judaism +1 prophet = Christianity, +another prophet = Islam.

**The punishment, of course, is not defeat, but being cast away from the one he adored (God).  As he fell to the depths of hell, a feather parted from his wing.  As it fell to earth, it became Satan's first gift to man: Freedom.