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.