Satanism is a group of religions that is composed of a diverse number of ideological and philosophical beliefs and social phenomena. Their shared feature include symbolic association with, admiration for the character of, and even veneration of Satan or similar rebellious, promethean, and liberating figures. Generally, those Satanists who believe in the Judeo-Christian concept of Satan are linked into the belief system of today's Judeo-Christian religions, as they believe in the same theology presented in the Bible.
Satan, also called Lucifer by many Christians, first appears in the Books of Chronicles provoking David to take a census of Israel. In the Book of Job he is called "the Satan" (meaning "the accuser") and acted as the prosecutor in God's court. A character named "Satan" was described as the tempter of Jesus in many of the Gospels of early Christians. Christianity and Islam typically regard Satan as the adversary or enemy, but extensive popular redactions and recompositions of biblical tales have inserted his presence and influence into every aspect of adversarial role back to the Creation and Fall. By Christians and Muslims especially, the figure of Satan was treated variously as a rebellious or jealous competitor to human beings, to Jesus, and characterized as a fallen angel or demon ruling the penitential Underworld, chained in a deep pit, wandering the planet vying for souls or providing the impetus for all worldly travesty.
At points during the development of the Christian religion, Satan became its outspoken enemy, and this served the interests of all those who would use this to their advantage, inclusive of those who fashioned or recomposed the mythos of Satanism. Particularly after the European Enlightenment, some works, such as Paradise Lost, were taken up by Romantics and described as presenting the biblical Satan as an allegory representing a crisis of faith, individualism, free will, wisdom and enlightenment. Those works actually featuring Satan as a heroic character are fewer in number, but do exist; George Bernard Shaw, William Blake, and Mark Twain (Letters from the Earth) included such characterizations in their works long before religious Satanists took up the pen.
Although the public practice of Satanism began in 1966 with the founding of the Church of Satan, some historical precedents exist: a group called the Ophite Cultus Satanas was founded in Ohio by Herbert Arthur Sloane in 1948. Inspired by Gnosticism and Gerald Gardner's Wicca, the coven venerated Satan as both a horned god and ophite messiah.
A particular antique Norwegian grimoire, in contrast to other Christian-oriented magical texts which describe Satan as an inferior spirit to be enslaved, contains a spell wherein the magician is instructed to renounce God and the Holy Spirit, and "completely swear to Lucifer, ruler of the Dark Abyss". The text itself claims to be originally from a manuscript in Wittenberg, similar to the many occult chapbooks pseudonymously ascribed to Doctor Faustus.
There was also a late 17th century French moral panic against alleged satanism during the Poison Affair (1675–1682), which occurred during the reign of Louis XIV and dealt with accusations of widespread poisonings, infanticide and forgery, presided over by an alleged satanic social network, which had no actual substance but reflected the aforementioned pre-Enlightenment popular religious anxieties.
Satanist groups that appeared after the 1960s are widely diverse, but two major trends are Theistic Satanism and Atheistic Satanism. Theistic Satanists venerate Satan as a supernatural deity. In contrast, Atheistic Satanists consider themselves atheists, agnostics, or apatheists and regard Satan as merely symbolic of certain human traits. This categorization of Satanism (which could be categorized in other ways, for example "Traditional" versus "Modern"), is not necessarily adopted by Satanists themselves, who usually would not specify which type of Satanism they adhere to. Some Satanists believe in God in the sense of a Prime Mover but, like Atheistic Satanists, still worship themselves, due to the deist belief that God plays no part in mortal lives.
Source: Wikipedia, "Satanist", available under the CC-BY-SA License.