Single-Precept Religion

In Real Life Religions tend to be built up of a vast number of different things and can take such a variety of forms that it can be very hard to pin down exactly what the word 'religion' means. As such, when someone needs one for a story, it can be very easy for them to to throw together any old thing and call it a religion. They reason that as long as it looks the part people will fill in the details for themselves. This generally leads to a lot of works featuring religions that look an awful lot like Christianity, Buddhism, or similar, with what those people believe roughly copied, though, in some cases, the writer will chose to create their own from scratch to suit their needs. Either route can result in vast and detailed histories and belief systems that are complex enough (or at least seem to be), to draw us in and immerse us in the writer's vision.

And then there are these ones.

A Single-Precept Religion is one where the writer created the look of a religion, but none of the substance. They may be grand and religiony-looking, but if you actually stopped one of the adherents and asked them what they actually believe....they can't really tell you. Or, if they can, the entire totality of their beliefs can be written on the back of a matchbook. Of course, they could be members of a Mystery Cult, in which case initiates actually won't know what they have gotten themselves into and the inner circle won't be keen on telling you (but that is another trope entirely).

In this instance they don't know because their religion doesn't actually have any beliefs, or, if it does, there are very few of them and they may actually sound like a secular secret society or be more akin to mysticism.

Subtrope of Artistic License Religion. Related to Fridge Logic.

Film

 * Bill and Ted's Excellent Adventure: The (future) Church of Bill and Ted has but one single commandment "Be excellent to each other and party on, dudes".

Literature

 * This is played with in Master of Space and Time by Rudy Rucker. One of the main characters basically wishes up a door to a parallel world where he can have an adventure. The world is controlled by a cult run by The Puppet Masters-like slugs. The cult has three teachings, God's Laws, which are "1. Follow Gary. 2. Be Clean. 3. Teach God's Laws". One character describes it as "A thought virus. A parasitic system that propagates itself."

Live-Action TV

 * Doctor Who features the Silence, which is basically a vast secret society dressed up as an oddball Christian sect (or, more accurately, mystery cult), complete with monks. It features one single solitary belief: that "". That's it. Why do they call it a religion? No idea.
 * On Dinosaurs the Elders start looking for a belief system simple enough to be understood by the dumbest individual. The winning entry is one based on the world being created by a potato.
 * The Bajoran religion on Star Trek: Deep Space Nine went into great detail about the customs and traditions of the Faith of the Prophets, but there was never much in the way of actual beliefs and tenets other than "Prophets Good, Pah-Wraiths Bad, Sisko Awesome."

Tabletop Games

 * Warhammer Fantasy Battle: The religion of the thunder god Tor has but one commandment: Don't stand under a tree in a thunderstorm.

Video Games

 * The Church of the Holy Light in World of Warcraft. People follow it like a deity and it's set up rather like Catholicism, but the actual beliefs of the religion are never really elaborated upon in the game.
 * The Chantry in Dragon Age is an unusual example. It has immense detail in its history, hierarchy, style and so on, but its actual teachings are pretty much limited to "Magic exists to serve man, not to rule over him." This seems rather odd in a role playing game which often asks the player to express either devotion or disdain for the chantry, despite knowing virtually nothing about it beyond it being the religion of the land.

Western Animation

 * When Bender is being worshipped by tiny aliens in the Futurama episode "Godfellas", he issues only one Commandment: God Needs Booze.