Only the Knowledgable May Pass

Perhaps you know the truth of how the city fell, three centuries ago. Perhaps you know the secret ritual of White Magic -- or Black Magic. Perhaps you know the way through the labyrinth. Perhaps you know Things Man Was Not Meant to Know. Perhaps you just know the secret handshake, or that The Password Is Always Swordfish. But if you can prove it, any of these are proof that you should be taken seriously: you are in the in-group, of those who know this esoteric knowledge.

Then, this can mean they want to be rid of you, or exploit you, and there is the little matter of figuring out who would understand if you say it. Heaven help you if you accidentally learned it; you will find yourself knee-deep in trouble before you can blink.

In The Infiltration, it is a frequent point of failure -- but if you do have the knowledge, it lets you in entirely.

Unlike Only Smart People May Pass, you can not figure this one out from the clues; you have to have learned it. (Occasionally, the Genius can manage to figure it out, but it will always take him serious time and effort, and it's not always possible even for him.) The Ancient Tradition, the Ancient Conspiracy, etc. take it very seriously.

Compare Trust Password, Only the Worthy May Pass, Only the Pure of Heart.

Fanfic
"Kage: After battle... Soi Fong: Many new ghosts cry. The solitary old man... Kage: ...worries and grieves. (Pause) To many places, communication is lost. I sit straight at my desk but cannot read my books for grief."
 * In Winter War, while meeting in an occupied city, Soi Fong and one of her Onmitsukidou use lines from a Tang Chinese poem as a password, switching off in the middle of lines and then skipping to the end. While someone sufficiently learned might know the poem, they probably wouldn't know when to switch off, or recognize what was going on based on the first phrase.

Film

 * In Monty Python and the Holy Grail, there was a bridge nobody could pass without correctly answering the guardian's questions. Those who failed were thrown down the abyss. King Arthur broke the curse when he was asked a question about sparrows and asked which kind of sparrows. Not knowing the answer, the guardian was Hoist by His Own Petard.
 * In Frankenstein: The College Years, Doctor Lipzigger's computer security demands, in addition to a keycard he bequeathed to the protagonists, the answers to two questions: One about chemistry, the other about Star Trek. It was meant so that only the protagonists may unlock it.

Literature

 * In Guards Guards, The Elucidated Brethren of the Ebon Night have a long string of passwords (as does every other secret society in the area, oddly enough at least some of them have the same ones, meaning they have to help each other's members to the right meeting place).
 * Dune: Lady Jessica is able to gain acceptance among the Fremen by using phrases planted in their culture by the Missionaria Protectiva (which manipulates religious beliefs to benefit the Bene Gesserit).
 * In Harry Potter, students in Hogwarts need a password to enter their dormitories. (With the exception of the students in Ravenclaw, who use a riddle -- Only Smart People May Pass.)
 * Fridge Logic: What's stopping a Gryffindor or Slytherin who can figure the riddle out from getting into Ravenclaw dorms when they shouldn't?
 * Basic courtesy? It's a dorm, not a bank vault. Besides, its actually pretty tricky to find the entrance to any given dorm in the first place if you don't already know.
 * Doubly subverted in Septimus Heap, since while the DoorKeeper of the House of Foryx doesn't allow Jenna and Beetle to enter even after they have resolved the puzzle that is The Right of The Riddle, they eventually get into the house with violence.

Live-Action TV

 * In Doctor Who, when the Doctor meets up with the amnesiac Brigadier, teaching at a boys' school, the Brigadier's attitude undergoes an abrupt change when the Doctor mentions UNIT -- he still doesn't know him, but anyone with sufficient secret clearance to mention it ought to know better than to talk like that.
 * On No Ordinary Family JJ proves he belongs in the Smart People's Club by citing its latitude & longitudinal coordinates.

Tabletop Games

 * Call of Cthulhu. In several published adventures, using your Cthulhu Mythos knowledge when speaking to cultists allows you to pass yourself off as one of them, which can considerably increase your life expectancy.

Webcomics

 * In Impure Blood, Dara uses it twice:
 * On Roan: How do you know the sacred oath of my people?
 * On Caspian: What servant girl knows the credo of the Watchers?
 * In Girl Genius, Violetta inverts it, thinking that Moloch's ability to get into the rafters shows he must have been trained or have some secret knowledge. He assures her that he knows he doesn't want to get munched on by the thing below them.
 * In Erstwhile, the prince questions the bride about things during the ceremony and says if she doesn't know them, she's not his true bride.

Western Animation
"Robot #1: Administer the test. Robot #2: Which of the following would you most prefer? A: a puppy, B: a pretty flower from your sweety, or C: a large properly formatted data file? Robot #1: Choose! (Leela and Fry whisper) Fry: Uh, is the puppy mechanical in any way? Robot #2: No! It is the bad kind of puppy. Leela: Then we'll go with that data file! Robot #2: Correct. Robot #1: The flower would also have been acceptable. Robot #2: You may pass."
 * In the Futurama episode "Fear of of Bot Planet", two characters in disguise must prove they are robots by passing such a test:

Real Life

 * In medieval times, masons needed a way to know whether other masons really knew their stuff, since they traveled a long way on jobs, failures of skill could be disasterous, and it was really hard to check with those who taught them. Their solution was secret rituals -- only a skillful mason would be taught them, so knowledge proved his skill. (The true origin of the Freemasons.)
 * To anyone who has ever forgotten a code or password to access an account, this trope applies. Painfully.