fanatic

Adjective

• marked by excessive enthusiasm for and intense devotion to a cause or idea
  • rabid isolationist
    

Noun

• a person motivated by irrational enthusiasm (as for a cause)
  • A fanatic is one who can't change his mind and won't change the subject--Winston Churchill
    

Related words

• Synonyms: 
  fanatical , overzealous , rabid , fiend