diehard

/D AY1 HH AA0 R D/
noun
  1. 1

    A person with such an attitude.

adjective
  1. 1

    Unreasonably or stubbornly resisting change.

  2. 2

    Fanatically opposing progress or reform.

  3. 3

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

    โ€œFor a Roman Catholic teacher, he sure is a diehard fundamentalist.โ€

Source: https://en.wiktionary.org/wiki/diehard