noun
fanatic; zealot
Refers to a person who holds extreme, unquestioning beliefs, often in a religious or ideological context. Can be used neutrally or negatively.
彼は宗教の狂信者だ。
He is a religious fanatic.
狂信者は自分の考えが絶対に正しいと信じている。
Fanatics believe their own ideas are absolutely correct.
信者 is a general term for a believer or follower, without the extreme connotation of 狂信者.
Compound of 狂 (kyō, 'crazy') + 信 (shin, 'belief') + 者 (sha, 'person'). The word literally means 'crazy believer'.