a person who believes in the existence or truth of something, especially somebody who believes in a god or religious faith 信徒
Idioms
be a (great/firm) believer in something
to believe strongly that something is good, important or valuable 坚信（或极力推崇）某事物的人 I’ve always been a great believer in getting as much sleep as possible. 我一直坚信要尽可能多的睡觉。