of course, we also need to understand Satanism in its original form, which is a philosophy, not a religion. in that way it's actually similar to Buddhism or Confucianism, the early forms are life philosophies, not religions.
and then of course there are modernist variants of Satanism that incorporate Satanist philosophies into a religious foundation, such as the Temple of Set, the Order of the Nine Angels, The Church of Satan, The Misanthropic Luciferian Order, The Order of Mars, Ordo Templi Orientis, etc.
Marilyn Manson is a Satanist, but he doesn't believe in the existence of Satan as a physical entity. the form of Satanism he ascribes to is the philosophical approach to life, which is more than the mere Law of Thelema ("Do What Thou Wilt"). Satanism as a philosophy, and many of the religious variants of Satanism, encourages people to fulfil their potential. to gain knowledge, share knowledge, think critically and not to blindly accept what people tell you unthinkingly. it encourages health. it encourages the celebration of life and existence. some may think this is an excuse for hedonism. the Satanist is, in fact, not a hedonist because hedonism is wasteful of our limited existence.
[edited : soft]