doctrinaire

/ˌˈdɑktrəˌnɛər/

Other forms: doctrinaires

You've doubtless met someone doctrinaire at some point. You know them by their complete unwillingness to accept any belief other than their own.

If you're familiar with the noun "doctrine" — a formal idea or system of belief — you'll have no problem with the adjective doctrinaire. It's a just a way of describing a person or group of people who are set in their ways. Parents can start out doctrinaire, but children soon force them to be flexible in how they bring them up.

Definitions of doctrinaire

1. noun

   a stubborn person of arbitrary or arrogant opinions
   synonyms: dogmatist

   type of:

   drumbeater, partisan, zealot

   a fervent and even militant proponent of something
2. adjective

   stubbornly insistent on theory without regard for practicality or suitability
   synonyms:

   informative, instructive

   serving to instruct or enlighten or inform