A Mythology Is True

A fantasy setting with the premise, or a premise, that one particular mythology is an approximation of the truth, usually with some plot-relevant differences. This is often a mythology associated with a mainstream religion.

If the mythology is true in the setting because the author actually believes it is true, however, this does not count. This only applies to real-world belief systems in fictional settings. Fictional religions being true does not count. If this is done to more than one mythology in one setting, then it is a Crossover Cosmology. If all of the stories are right, it becomes All Myths Are True.

Comic Books

 * The Chronicles of Wormwood to Christianity.
 * The Tintin (Comic Book) series' The Seven Crystal Balls and Prisoners Of The Sun have a real-world voodoo curse in effect, albeit attributed to Incas.

Films -- Live-Action

 * The film Constantine does this for catholicism. In contrast to the original Hellblazer comic book where All Myths Are True.

Literature

 * Expecting Someone Taller, by Tom Holt, to the Germanic pantheon.
 * Good Omens to Christianity (mostly of a heavily Milton-influenced variety)
 * Also by Neil Gaiman (but not Terry Pratchett) was Anansi Boys, focusing on the concept that West African mythology (and all named spiders and panthers that go along with it) do exist in the modern day. Unless they're dead.
 * Anansi Boys takes place in the same continuity as the earlier American Gods, which makes the same assumption about all mythology-some gods have died, others have wandered off and forgotten who they were, but all of them existed at one point, with greater or lesser degrees of being anything like the way myths paint them.
 * JRR Tolkien's early drafts of The Silmarillion, published in Lost Tales I&II, are a clever inversion of this: Through a series of events and battles that are echoed mainly in Norse and Finnish mythology  This was toned down significantly to the point of near-abandonment in later drafts, though the fading of the Elves and the gradual dominion of humans remains a prevalent theme. Also impossible to get rid of completely: Some cunning linguists pointed out that a huge number of Tolkien's Eldarin word roots are built to act as predecessors of reconstructed Indo-European, theoretically transporting the idea of early humans using language they were taught by the Elves into the real world. See Faramir's quote on all speech of the world being Elvish in origin.
 * Percy Jackson and The Olympians to ancient Greek Mythology.
 * The Kane Chronicles to Egyptian Mythology
 * Also, the Roman gods exist as well, just as alter-egos to the Greek ones. There's evidence that the two series exist in the same universe, and that all pantheons exist (ignoring each other for the most part) creating a kind of Fantasy Kitchen Sink.
 * The two have a Shared Universe according to Word of God so actually they fall under All Myths Are True.
 * Bring Me the Head of Prince Charming, by Robert Sheckley and Roger Zelazny, to Christianity -- or its Theme Park Version. Mostly Played for Laughs.
 * Welsh mythology in The Dark Is Rising. More or less.
 * Jonathan Strange and Mr. Norrell at the very least presents as true English legends concerning The Fair Folk, and Merlin is referenced as being a real person. At least, because in the story collection The Ladies of Grace Adieu and Other Stories, there is a Crossover with Stardust, and Neil Gaiman at least has placed Stardust within his All Myths Are True 'verse.

Live Action TV

 * Supernatural is an interesting case which can't decide whether it's this trope or All Myths Are True. Earlier seasons seem to imply the latter, with the characters explicitly stating that "almost all cultures" have lore of some kind about the Monster of the Week, with only slight variations. However, later seasons seem to run on the basis of Christianity (with God, angels, and Lucifer).
 * A season 5 episode even addresses this issue with a gathering of gods from other/more ancient religions being mad that the Judeo-Christian apocalypse is going to end the world instead of their own religion's version of the apocalypse.
 * However, the fact that seems to suggest that Christianity still comes out on top.
 * It seems that neither the angels, demons, or pagan gods are truly "gods" but rather creatures that have been worshiped as such over time. Some angels mentioned that they came from elsewhere and took over from the old gods; Death claims to be as old as God. Neither concept is from any form of real Christianity.

Video Games

 * God of War to Greek Mythology.
 * Although Word of God has said it's a case of All Myths Are True - every other god and pantheon exists and Kratos is going to kill them all.
 * Touhou and Japanese mythology. If you ignore the earlier games, anyway.

Web Original

 * Lampshaded in Yu-Gi-Oh the Abridged Series.

Western Animation

 * In South Park, the Mormons are right.
 * In the episode "Best Friends Forever", that changed when they need more soldiers to join in the battle against Satan's legion of Hell, and Mormons aren't fighters. Considering it's South Park, things went back to the way it was not that soldiers aren't needed.