Satanism

Satanism is a ferocious religion based on materialism, the empowerment of the self and the ego, the carnal realities of animal life, the questioning of social taboos, the ridiculing of most other religions, and the promotion of tough social justice. Satan is not real but is the most ideal and accurate symbol of reality and nature: The world is full of violence, suffering, stress, striving and death - the life cycle itself requires all these things. It is clear that no symbol of 'goodness' or 'light' can embody it all. The vast, uninhabitable darkness of the Universe, with galactic cataclysms rendering huge destructions on huge scales, means that only Satan can embody the true state of everything that is. Satanists tend to use all the symbols of darkness and evil. There is no heaven, no hell, no afterlife, no angels or demons: there are no gods or saviours apart from ourselves. But if there was a god, it would surely be utterly evil.

The Church of Satan was founded in 1966 by Anton LaVey and as of 2001 its Black Pope has been the powerful and articulate Peter Gilmore. Satanism as religion is highly skeptical, rationalist and philosophical, demanding evidence and strong argumentation, but is otherwise open-minded on magic and some parts of the supernatural. Satanists are often bold, intelligent, demanding; but are also often subtle, cautious, hidden, while others still are outrageous, aggressive, angry and puzzling. It is a religion of fire, drama, depth and forceful progress.
I maintain a large website on Satanism and the above is taken from the 2-paragraph description of LaVey Satanism from "The Description, Philosophies and Justification of Satanism" by Vexen Crabtree (2010). Its full menu is

1. Satanism in Two Paragraphs
2. Some Opening Declarations
3. Classifications, Keywords and Sociological Definitions of Satanism
4. The Satanic Bible
5. The Symbol of Satanism: The Baphomet
6. External Links

A central argument of Satanism is that the evidence found in the real-world suggests that "if there is a God, it is evil":

“
The existence of such large quantities of suffering, despair, pain, of natural disasters such as earthquakes, of the death of the unborn and the immense suffering of lovers & kind-hearted people means that god is evil and intentionally creates life in order to create suffering. That all life exists in a food chain means that life is completely tied to death, and such a barbaric biological cycle could only have been made by an evil god. Also, that such a god appears not to exist, or actively hides itself, is a source of confusion, conflict, war and stress and is again more likely the antics of an evil god. Given the state of the natural world, it is impossible that a good god exists. It is more likely that an evil god exists, but, it is sensible to assume that there is no god of either type. Even if there is not a god of either type, as the dominance of death and violence in the natural world, a result of nature being abused by life and not being designed for life, I think the evil symbol of Satan is the best representative of the state of reality and the universe, whether or not an actual evil god exists.
If God did exist and was evil, it would undoubtedly lie and tell everyone it was a good god and that it loved them. It would create maximum confusion by preaching multiple conflicting religions. It would create heaven and make it hard to get to in order to tease and torture people into making their own lives hell. As all of those things happen, if there is a God, it is doing the things an evil God would do!
Once I recognized and accepted this state of affairs and adequately called myself a Satanist, I could concentrate my life on happiness, love, stability and peace. Because I know and understand that death always wins, that life is temporary, I waste no time on short-term whims that reduce my quality of life, or of those around me, and I waste no time with spiritual pipe dreams. Recognizing Satan as the personified meta-figure of reality is self-affirming, life-affirming, positive, honest and clarifying.”

"God Must Be Evil (If It Exists): 5. Conclusion" by Vexen Crabtree (2003)

There are different denominations of Satanism which have spun off from the original Church of Satan, but, most forms fit into one of the following categories:

1. Non-Dualistic Satanism
   1. Modern LaVey Satanism: Satan is a Concept
   2. Satan is the Dark Lord: There is No God
   3. If There is a God, It Is Evil
2. Dualistic Satanisms
   1. Satan is the Victor: The Evil God is the Strongest
   2. The Powerful White Light God is Evil: Satan is Righteous
   3. Classical Gnosticism
3. White Light Religion Causes Imbalance and Evil
4. Satan is the Champion of the Alienated