One True Faith

The phenomenon in which a work of Science Fiction or Fantasy has only one religion (The Church) in it; furthermore, there are never any factions of it, or different interpretations of its belief.

This is related to Law of Conservation of Detail. The lack of other faiths doesn't particularly break Willing Suspension of Disbelief, so why bother? Having said that, it's inherently unrealistic; our planet has seven or ten major religions in the modern day alone (some founded individually and others spun off from existing ones), and other religions have come to prominence and since died out. But no one ever questions this when it happens, making it either an Acceptable Break From Reality or something too controversial to touch with a ten-foot pole. It is most justified in works where much more is known about the spiritual world than in Real Life, and there's lots of evidence to back it up to the point that not believing in this faith would make one a Flat Earth Atheist (though that still doesn't explain the lack of factions; there is no hair so fine that people won't try to split it).

Ignore Outgrown Such Silly Superstitions.

As discovered during the YKTTW process, this trope is more of a Sliding Scale than a straight trope, since there are numerous aversions and subversions. We'll go in order.

Literature

 * The Sword of Truth
 * Not true. There are several religions in that series and one of them has several different factions who violently disagree. Played straight in that each racial or cultural group only has one religion.
 * The Wheel of Time, arguably. See "subversions" below.

Video Games

 * Averted in X-2, where the former Yevonites have divided into the Youth League and New Yevon. The major bone of contention is the speed at which technology should be incorporated into daily life.
 * In Tales of Symphonia, two entire worlds share the same religion with only minor cosmetic differences. Justified by the Big Bad having created and actively running both. Although the village of Mizuho has its own gods.

Anime and Manga

 * The Familiar of Zero played with this, in the fact that humans have one religion, based off on Crystal Dragon Jesus version of Christianity in the Fantasy Counterpart Culture of Vatican, Romalia. However, it is not the only religion per se, as the Elvens follow a different one. This becomes a plot point in the third season.
 * A Certain Magical Index has just every religious denomination that's present in the real world in its world as well. They're all true, and each has its own magic.

Literature

 * Discworld, with its proliferation of Physical Gods
 * A Song of Ice and Fire, which has featured five in four books alone.
 * His Dark Materials, even though The Church gets all the attention
 * Wicked, at least in the original book
 * In Orcs, not only are there several religions (mainly a monotheist and a polytheist one), but they are very important in the plot.
 * Ender's Saga where matters of ensuring freedom of religion are plot-significant at some point.
 * In the Left Behind series during the Tribulation, there's the pantheistic mishmash of all world religions called Enigma Babylon One World Faith as the official one-world religion, but there's also Christianity (as defined by the books' authors and the Tribulation Force characters), Judaism, and Islam (which in the book series ends up being a minority religion), which all become illegal to practice even when Enigma Babylon is replaced by the Luciferian-type state religion of Carpathianism around the midway point of the Tribulation. In the Millennial Kingdom, Christianity becomes the official one-world religion, though dissidents are permitted to practice the beliefs of the Other Light despite the fact that (1) it's totally in error when it comes to believing Satan is going to defeat God and Jesus by the end of the Millennium and (2) those who remain unbelievers in God and Jesus Christ by the time they reach 100 will instantly die and go to Hell.

Tabletop Games

 * Exalted: While the Immaculate Order would certainly love to be the One True Faith, the plain fact of the matter is that human nature and the cosmology of the setting works against it. As it is, they only really have this status on the Blessed Isle, and only there because they ruthlessly suppress any hints of non-Immaculate leanings.

Video Games

 * Xenogears features two distinct major religions, (very) briefly hints at a third in one scene. Most of the beliefs of the organized religions, however, seem to be based upon a similar set of concepts.
 * World of Warcraft has multiple religious faiths, many of which are diverse from one another.
 * In the Alliance, the humans, along with the dwarves and the draenei, practice a Crystal Dragon Jesus religion known as the Holy Light. The exceptions are the night elves, who worship a moon goddess known as Elune, possibly the Gilnean worgen, former humans who were brought to the Alliance by night elves (though their religious practices are almost unknown), and the gnomes, who have for the most part completely forgone religion in favor of science.
 * It's noted in the tabletop roleplaying game that Elune is the only true actual deity in the game world, though that sounds like Night Elven propaganda to me.
 * It should be stated that the Holy Light is non-theistic (that is, it has no godhead) and allows for some degree of syncretism.
 * That said, earlier games make references to a more theistic bent, and references to a deity by worshippers of the Holy Light have been seen within World of Warcraft. Given the amount of turmoil that has taken place over the course of the games, it's entirely likely that the Church has undergone theological, philosophical and metaphysical developments.
 * The Horde is split up for the most part. Orcs, as a whole, have a shamanistic heritage. Almost all trolls worship their multiple Loa spirit gods. The tauren worship their tribal deity known as the Earthmother. Forsaken undead have some sort of religion involving the shadow, themselves and the light(to a lesser extent). Blood elves follow the Holy Light. Its unknown what goblins worship but they can be priests.
 * In addition, there are many other races that worship other deities: members of the Cult of the Damned worship the Lich King, many corrupted followers of the Old Gods exist, and there are many others.
 * Breath of Fire II, which sports the benevolent but mostly impotent Dragon God, the minor earth deity Namanda and the  Church of St. Eva.
 * Repeated in III, with the God of the Urkans  and the Dragon God Ladon, who still has a few shrines as the "god of travelers" and serves as a Master.
 * The Elder Scrolls. Morrowind, for example, has the Imperial Cult, the Dunmer Tribunal, the native Ashlander religion, and the Dagoth Ur Religion of Evil.
 * However, every culture in the game has basically the same creation myth. They just interpret it differently.
 * The Ordinators random bark of "Three gods, one true faith" has nothing to do with this trope.
 * Vagrant Story, with two
 * La Pucelle, technically: although the story centers on members of the titular church, there is another one who doubles as its theological rival, enemy and Church of Evil.
 * Arc Rise Fantasia has three, although two (North and South Noireism) are kind of variations of each other.

Webcomics

 * Get Medieval, with both earthly religions and alien ones.

Real Life

 * But of course. Also, nearly every faith is heavily fragmented into several different sects.

Film

 * Star Wars (Jedi vs. Sith at the very least. The Expanded Universe features enough "alternative" religions to make the franchise an aversion, but we're ignoring those for the moment.)

Literature

 * Lord of the Rings, in which certain cultures (or subcultures) greater more emphasis on their favorite members of the pantheon
 * Omnianism in Discworld, going through endless schisms after the last prophet, Brutha in Small Gods
 * Lois McMaster Bujold's Chalion universe has two religions who are bitter rivals, despite the fact that the only difference in their philosophies is how one particular being fits into the pantheon.
 * David Weber's Safehold series is the story of one world's transition from a single monolithic religion to a deeply divided global religion - but it's still the same religion, just divided. The second and third novels in the series are called By Schism Rent Asunder and By Heresies Distressed, which are a fairly accurate description of the fate of the Church of God Awaiting.
 * The reason for the single religion is that it was achieved artificially through massive brain washing and by nuking any opposition from orbit. We do not know what the religious makeup of the original colonists was. Other religions might have popped up over the centuries but they would have been mercilessly wiped out by the Church.
 * The Wheel of Time, like Star Wars, has one single monolithic belief system for the whole world - but it's dualistic, and the darker side has a few adherents here and there, and they're the antagonists. Well, the main antagonists. So that darker side might be considered a schismatic completely separate religion or a part of the same religion with different ways of observing it.
 * As typical of Robert Jordan, every side also has different factions: for example, the Children of Light and the Aes Sedai both worship the Light and the Creator, but have a very different view of the One Power... Masema's followers also behave like a fanatical cult, putting Rand on the same level as the Creator.

Live Action TV

 * Doctor Who: In the future the army (human army that is) is run by the Church and among the Clerics there are mentions of both Anglican troops and of a Papal mainframe.

Tabletop Games

 * Dungeons & Dragons
 * Warhammer 40,000 has enough cults of the God-Emperor that almost everyone is, indeed, a heretic.

Western Animation

 * South Park had a two-parter set in the future where everyone has become an atheist. However, the atheists have split into many factions, fighting wars just as bloody as the religious wars of the past, over.

Comic Books

 * Artesia has multiple religions (the Old Religion of Yhera, the faith of the Divine King, and the mystery cults, among others), but they all fit into the same cosmology, many gods are worshipped in both religions, and the Phoenix Court of the Empire worships the Divine King but also pays respect to the gods of the Old Religion.

Literature

 * Honor Harrington
 * Let's see, we've got the formerly Space Amish Church of Humanity Unchained, with its own internal factions, and its extremist splinter The Faithful, the Methodist-descended Third Stellars(which has practically every congregation (Or even every person) being their own faction), along with a 2nd Reformation Catholic Church, a new Judaism faction, and passing mentions of most other current major religions.

Live Action TV
"Cmdr. Sinclair: shaking hands or bowing or giving the appropriate gesture This is Mr. Harris. He's an atheist. Father Cresanti, a Roman Catholic. Mr. Hayakawa, a Zen Buddhist. Mr. Rashid, a Moslem. Mr. Rosenthal, an Orthodox Jew. Running Elk, of the Oglala Sioux faith. Father Papapoulous, a Greek Orthodox. Ogigi-ko, of the Ebo tribe. Machukiak, y Yupik Eskimo. Sawa, of the Jivaro tribe. Isnakuma, a Bantu. Ms. Chang, a Taoist. Mr. Blacksmith, an aborigine. Ms. Yamamoto, a Shinto. Ms. Naijo, a Maori. Mr. Gold, a Hindu. Ms.... Fade to Black"
 * Star Trek, which features religious Planet of Hats (Bajor), Outgrown Such Silly Superstitions (while Roddenberry was alive) and religious proliferacy (after he'd passed on)
 * Babylon 5: In the episode "The Parliment of Dreams" during a cultural exchange week each of the alien races put on some kind of demonstration of their religion. Every alien species only has one religion, but what will the Earth do? Finally it's Earth's turn.

Video Games

 * Dragon Age: Origins: more than one religion overall, but you only get one religion per race - there is no sense that any particular person could ever be a different religion from the one they are. The human religion, that of The Maker and the Prophet Andraste has some subdivisions, but these are clearly marked as 'real religion' and 'evil cult' so don't really represent religious diversity. The narrative itself (and the Warden, if you choose) can be read as having a subtle athiestic bias.
 * Notably averted in Orzammar, where you find a dwarf cleric of Andraste who is genuinely good; should you choose to help him, you see why there's only one religion per race.
 * There's also a subversion that gets a bit more attention in Dragon Age II, where the Tevinter Imperium has its own schismatic Chantry that's under the control of the mages, rather than the other way around. Fenris calls it a scam to keep the magisters in power, but we don't actually see the Black Divine or any of his people.
 * Also, if you read the Codex, the reason there's only two versions of the Maker's worship is because the Chantry takes it very seriously, aggressively wiping out variations of the faith. Kind of like the Catholic Church used to, only with more success. It's even part of the religion that the Chant must be recited perfectly, with no variations. However, despite all of this, the occasional underground offshoot manages to thrive.