In its original appearance in the old Israelite faith tradition, "Satan" was actually a title -- ''haSatan'', or "the Satan". (The root ''s-t-n'' in Hebrew means "adversary" or "opponent", and ''ha'' is the definite article.) It almost exclusively refers to the Evil Inclination, the counterpart to the Good Inclination, which are Judaism's equivalent to the [[GoodAngelBadAngel angel and devil on each shoulder]] (i.e., it is an internal rather than an external influence on human action). The word was also used for mortal functionaries -- often what would be called investigators or [[TheSpymaster spymasters]] today -- in the courts of earthly kings. However, on some occasions it also seems to refer to that spirit or angel in God's court who would test or question the faith of mortals. An analogy frequently used in rabbinical literature to describe this state of affairs is that of a prostitute a king hires to try to [[SecretTestOfCharacter seduce his son]]: the prostitute, no less than the king, wants the son to pass the test and resist her advances, but is still obliged to work as hard as she can to make him fail because that's what the king wants. In modern times, a more familiar analogy would be the professional OPFOR (opposing force) used in militaries, or -- from ''Film/WillyWonkaAndTheChocolateFactory'' -- [[spoiler:the actor hired to pose as Wonka's business rival Slugworth and test the kids' loyalty to Wonka]].

However, autonomous and unambiguously evil spirit powers are also acknowledged in the Old Testament; for example, the gods of the heathen nations are said to be demons, some of them quite powerful; there also exists a "king of terrors" who lives in Hell and feeds on the souls of the dead. The prophets speak of a fallen cosmic ruler cast down in Hell, who was once a cherub, or angel of the highest order (though other interpreters think this is mere hyperbole describing a human king), and various books speak of a monstrous cosmic dragon (a sort of [[Myth/NorseMythology Jormungandr]] prototype) as God's most powerful enemy. All of these are traits which Christian theology ascribe to Satan, though in Old Testament times they were not yet explicitly linked to the name of ''ha-Satan''.

In late Jewish tradition, such as the Dead Sea Scrolls and the ''Martyrdom of Isaiah'', all of these intimations were interpreted as [[IHaveManyNames describing one entity]], and ''ha-Satan'' became simply Satan, the universal Adversary, as he came to be understood as the personal embodiment of active opposition to God. The ''Wisdom of Solomon'' (a late Jewish book, considered canonical by Roman Catholics and Eastern Orthodox Christians) describes him as responsible for the original Fall, while the Book of Revelation calls him "the ancient serpent," a name often interpreted to mean that he ''was'' in fact the tempting serpent in the Garden of Eden. This [[{{Pun}} revelation]], combined with his and the demons' war against ArchangelMichael and the angels in the same book, cemented his new identity as an ChaoticEvil FallenAngel in direct opposition to God.

The fully developed teaching of the New Testament presents Satan as an incredibly powerful, and absolutely malevolent, cosmic entity, far beyond human ability to fathom. While not a true [[TheAntiGod anti-God]] in the dualistic sense, he is portrayed in terms that suggest almost god-like power. He is said to control both the micro- and the macrocosmos, from the "elements of this world" to the "celestial bodies" and beyond; his lieutenant demons are called "rulers of the universe" (''Kosmokratores''); himself, he is frequently referred to as "the Lord of this world" and once, actually "the ''god'' of this world"; and indeed, it is written explicitly that "the whole world is under the control of the evil one." Further, the physical form he affects when making a personal appearance is beautiful and terrible as an "angel of light"; mortals (even devout Christians) spontaneously react to the sight of his lesser creatures, such as the Beast of Revelation and the Whore of Babylon, with wonder and almost worshipful admiration. Finally, he rules millions and millions of very powerful fallen angels, and can easily [[VillainOverride assume direct control]] of anyone in the world who is evil (or even just suffiently non-Christian, really), and/or imbue them with the powers of a minor PhysicalGod if he so chooses. Described in pop-culture terms, he is probably best understood as a ''very'' powerful DimensionLord or GodOfEvil: In most any pagan pantheon, he would probably be the GodOfGods. Fortunately for humanity, he lacks the absolute omnipotence and omniscience of the true Deity, which is why the final victory of good over evil is still assured, in spite of his cosmic near-supremacy.

The wider [[TheThemeParkVersion pop-culture conception]] of Satan is often rather different from the Biblical picture, tending, perhaps for dramatic reasons, to make him a much less formidable figure that mortal agencies can realistically combat and thwart. This version, too, rests on a long, though younger tradition. Medieval folktales often portrayed him as little more than a [[TheDevilIsALoser bumbling oaf]] of a demon, wandering the world trying to tempt the faithful, whom the lowest of common field hands could outwit if he kept his head about him. (Echoes of such humorously pathetic demons still abound; for example, in many of the works of Creator/CSLewis, though then of course in a much more sophisticated form.) As the Middle Ages faded into the Renaissance and beyond, Satan cleaned up his act somewhat -- or, rather, had it cleaned up for him. He became an [[WickedCultured elegant, educated]] [[ManOfWealthAndTaste figure]], able to mingle undetected with the [[BlueBlood aristocracy]]. Milton's portrayal of Satan in ''Literature/ParadiseLost'' even added a bit of true nobility to him and ([[MisaimedFandom unintentionally]]) cast him as an almost-sympathetic AntiHero. But he was still far less powerful than God -- constrained by corporeality and [[IGaveMyWord bound inextricably by the promises he made]], he could still be outwitted. Even so, Satan became more attractive as an antagonist. In many ways, this version of Satan is still popular today, as it provides a kind of comprehensible evil that the hero can face down and (ultimately and perhaps with help) defeat.

In several influential strains of modern Christian thought, Satan's immense cosmic agency and power are again emphasized, if they were ever really forgotten, and some of this has also bled through into much contemporary fiction. Almost as omnipotent and omnipresent as God, this old-new version of Satan can be broadly impersonal or [[ReligiousHorror terrifyingly intimate]], suiting him to [[CosmicHorrorStory Lovecraft-style horror]] where he can only be pushed away for a little while and never truly defeated, at least not until Armageddon itself. (And, probably not coincidentally, suiting him also to scaring the bejeezus out of and inspiring dependent paranoia in church congregations.)

A common belief (probably descended from Milton's "rule in Hell" line) is that Satan is in charge of Hell. This does not tally too readily with the orthodox Christian view, which involves Satan being eternally punished in Hell when Judgment Day comes. Could be understood as Satan simply being like the biggest and baddest convict in a maximum security prison: still punished and locked up (though Literature/TheBible does have him showing up on Earth from time to time), but still wielding AsskickingEqualsAuthority.
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