Shibboleth

A term, trait, belief, or action used to identify people belonging to the same group

Example:

The liberty of the citizen to do as he likes so long as he does not interfere with the liberty of others to do the same, which has been a shibboleth for some well known writers, is interfered with by school laws, by the Post Office, by every state or municipal institution which takes his money for purposes thought desirable, whether he likes it or not.
— Oliver Wendel Holmes, Jr. Lochner v. New York

He boldly challenged the most cherished shibboleths of American political thought…a systematic critique of the very principle of American democracy.
— S. T. Joshi, Mencken’s America (2004)

During the war Gramsci drew these concerns together in a vitriolic attack on the favourite shibboleth of prewar anarchism and socialism: Esperanto.
— Carl Levy, Libertarian Socialism: Politics in Black and Red (2012)

But maybe prayer is a road to rise,
A mountain path leading toward the skies
To assist the spirit who truly tries.
But it isn’t a shibboleth, creed, nor code,
It isn’t a pack-horse to carry your load,
It isn’t a wagon, it’s only a road.
And perhaps the reward of the spirit who tries
Is not the goal, but the exercise!
— Edmund Vance Cooke, Prayer, The Uncommon Commoner.

Etymology:

According to Judges 12 of the Old Testament, people called Ephraimites were unable to say “shibboleth” (a word meaning “flood”), pronouncing it “sibboleth”. This allows them to be identified and killed by enemy Gileadites:

And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; ⁶ Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand.

Judges 12:4-6