fanatic; zealot
Refers to a person who holds extreme, unquestioning beliefs, often in a religious or ideological context. Can be used neutrally or negatively.
He is a religious fanatic.
Fanatics believe their own ideas are absolutely correct.
Compound of 狂 (kyō, 'crazy') + 信 (shin, 'belief') + 者 (sha, 'person'). The word literally means 'crazy believer'.