diehard

Unreasonably or stubbornly resisting change.idiomatic

Fanatically opposing progress or reform.idiomatic

Complete; having no opposite opinion of anything in a particular topic of one's values; thorough of in one's beliefs.idiomatic

• For a Roman Catholic teacher, he sure is a diehard fundamentalist.

Strongly or fanatically determined or devoted.idiomatic

A person with such an attitude.idiomatic

• I should like my Scottish correspondent to see recent statements by leading officers of certain American railways which both have been among the most extreme diehards for steam, and also have developed the most powerful and outstanding stea