unbeliever; nonbeliever
Refers to a person who does not believe in a particular religion, doctrine, or ideology. Often used in religious contexts.
He is an unbeliever in Christianity.
We need an explanation that is easy for nonbelievers to understand.
異教徒 means 'pagan' or 'heathen', often used for followers of a different religion, whereas 不信者 focuses on the lack of belief itself.
Compound of 不 (fu, 'not') + 信 (shin, 'belief') + 者 (sha, 'person'). The reading ふしんじゃ follows standard on'yomi.