Interfaith Smoothie

This is when a constructed fictional religion is clearly a mix of any number of real-world religions. An author will often use this by combining various interesting bits of existing religions, belief systems, and philosophies, and changing the names and places to make the new religion fictional.

Differs from Crossover Cosmology in that this creates a 'new' cosmology from pieces of established idea systems. Popular in Space Opera and Science Fiction as representative of alien cultures. A subtrope of Ninja Pirate Zombie Robot. Can also be Truth in Television since there are few religions that don't share certain rituals or beliefs with other religions.

Anime and Manga

 * This seems to come up a lot in Anime fantasy settings, though it could (and oftentimes appears to) just be a cultural misunderstanding of Western ideas and philosophies while substituting familiar concepts (like Buddhism and Shintoism) into the knowledge gaps.
 * Also, sometimes in Anime, instead of All Christianity Being Catholic, you'll see a mix-mash of ideas from various Christian denominations all pooled into a single faith. This probably has a lot more to do with the aforementioned Critical Research Failure, though.

Comic Books

 * In "Tintin in America". It's a mixture of Judaism, Buddhism and Islam, they claim they were the fastest growing religion, and want Tintin to become a member of them.

Film

 * The Force, once described by Mark Hamill as "Religion's Greatest Hits!" The religion of the Force has strong elements of Buddhism, Jainism, Hinduism, Daoism and a bunch of other mystical traditions, with Christian symbology (the Jedi are Knights In Brown Robes and not for nothing is the hero named Luke).

Literature

 * The Thursday Next series has the Global Standard Deity, a church that openly and shamelessly mixes and matches elements of various faiths.
 * The end of the Pendragon series has Ravinia, in which people see life in the rest of the universe (Halla) for the first time. This is not a religion in itself, but something that attracts people regardless of religion into one group. On the other hand, it divides people based on social class.
 * The far future religions in the Dune series are either this or the Coca Pepsi Inc type. The Coca Pepsi Inc ones are the more numerous though.
 * The Videssos books, being chockablock with Fantasy Counterpart Cultures, have lots of these. The religion of Videssos proper (the fantasy analogue of the Byzantine Empire) looks a lot like Orthodox Christianity (with bishops, monks, ecumenical councils, schisms over variations in the Creed), but the dualistic belief system is much more like Zoroastrianism (two powerful gods, one good and one evil, at war). There are heresies with variant understandings of the war (Videssians believe the good god is sure to win, Khatrishers believe the two gods are perfectly balanced, Namdaleni believe the gods are balanced but you ought to act as if you're sure the good god will win). The main other empire starts out as practically-Muslim (with belief in a single God and four supreme Prophets), but ends up being dominated by a diabolist religion that worships the evil Videssian god.
 * In Life of Pi, the title character manages to be a practicing Hindu, Christian, and Muslim all at once, to the confusion of most of the other characters.

Live Action TV

 * The Klingon belief system seems to be an odd mish-mash of Shinto and Norse mythology, with a Messianic Archetype figure (Kahless) thrown in. Vulcan spirituality seems to have elements of Shinto ancestor worship within a predominant Buddhist philosophy, with 'logic' substituted for Dharma. The Bajoran faith which features prominently in nearly every episode of Star Trek Deep Space Nine may be the ultimate example of this trope, combining elements of all three Western monotheisms (Judaism, Christianity and Islam) together with Eastern Hindu/Bhuddist mysticism.
 * Wayism in Andromeda was this. It was mostly Buddhist 'peace in suffering' teachings, with a few other things thrown in. In universe it was created by a Magog who was taught the concepts of various religions to him by the human host he was spawned from.

Video Games

 * Solatorobo's Oshilasama seems to be one part the Buddhist-Shinto amalgam common to Japan and one part Functional Magic. Oh, and something about an evil dog-god who loses his powers if you turn his statues upside-down.
 * Oracle of Tao has a strange mix of Shintoism, Taoism, and Christianity. They call it Aiken (based on Japanese ai ken, not Clay Aiken). It's basically very heavily into nature and love (and ancestor worship).
 * In EV Nova, the Church of Krim-Hwa is this in-universe.

Web Comics
"“I’m the kind of Christian who only goes to church on Christmas and Easter, and spends the other 363 days at the mosque.” “… I don’t think that’s a thing.” “Our rabbi swears it’s legit.”"
 * The Church of Slag-Blah in Buck Godot: Zap Gun for Hire who are "militant agnostics" and celebrate a different religious holy day every day.
 * In an Xkcd strip, this discussion takes place:

Web Original

 * Sanshinto or Tritheism in Tasakeru is based primarily on Shinto, but has elements from Greek, Roman, and Norse mythology. The species' differing beliefs draw from everywhere., even, according to the author, the Cthulhu Mythos
 * The Chaos Timeline has the Indian Chandramoorthy develop his own religion, which combines elements from Hinduism, Islam, Catholicism and the classical Greco-Roman religion.
 * Gamzee's religion in Homestuck seems to be based around fundamentalist and Rapturist Christianity, with a little Islam for flavour (he gets very upset about seeing video depictions of his Messiahs), and perhaps with a little Judaism (his ancestor was responsible for the persecution of the Troll Jesus), mixed up with stoner/hippie culture, and then all applied to Juggalo fandom. A parody, obviously.

Western Animation
"Father Changstein El-Gamal: Dearly liked, we stand here before one or more gods, or fewer; to join this couple in pretty good matrimony. If anyone objects to this union, may they speak now, or forever hold their peace; or do something else."
 * Futurama plays with a somewhat joke-y version of this trope in the First Amalgamated Church, headed by Father Changstein El-Gamal. Supposedly created from the merging of major 20th Century religions - Christianity, Judaism, Islam, Hinduism and Buddhism as well as agnosticism, and the logo shows it. Differs from more serious examples in that part of the joke appears to be that the Church doesn't have even a semi-coherent belief system and mostly just tries to be as generically 'spiritual-ish' as possible.


 * The only thing you can definitively say about Reverend Lovejoy's church in The Simpsons is that it's some variety of Protestantism. In an episode where Bart and Homer convert to Catholicism, the Rev describes the One True Faith as being "the Western Branch of the Reformed Church of American Presbo-Lutheranism".

Real Life

 * Certain Unitarian Universalist congregations can end up like this. Since UUism rejects the idea of central dogma in favour of emphasising the value of spiritual community, nothing stops any individual from being a Christian, Jew, Muslim, Buddhist, Hindu or even atheist/agnostic and still a practising UU. It's not too far from the truth to joke that that UUs begin their prayers with "To whom it may concern..."
 * The proper term for this trope is Syncretism. As you can see, there are enough examples of it in Real Life.
 * In the 19th century, it was very common among scholars of world religions to gain a complete understanding of God by bringing the knowledge of all religions together to create a unified whole. While certainly admirable, religious authorities of all relgions where mostly unimpressed and didn't share the belief that other religions had anything to contribute to "their" already perfect models.
 * This (text: "Basics of religious cultures and secular ethics") was a reaction on the introduction of this experimental "obligatory facultative" school course -- which, obviously, managed to unite everyone, if only in condemnation of this offence.