Fanatic. : marked by excessive enthusiasm and often intense uncritical devotion <they’re fanatic about politics> Merriam Webster
Fanaticism is a belief or behavior involving uncritical zeal, particularly for a religious or political cause or in some cases sports, or with an obsessive enthusiasm for a pastime or hobby.–Wikipedia
A fanatic is one who can’t change his mind and won’t change the subject. Winston Churchill
Defined in psychological terms, a fanatic is a man who consciously over-compensates a secret doubt. Aldous Huxley
The fanatic is incorruptible: if he kills for an idea, he can just as well get himself killed for one; in either case, tyrant or martyr, he is a monster.
Emile M. Cioran