noun
unbeliever; nonbeliever
Refers to a person who does not believe in a particular religion, doctrine, or ideology. Often used in religious contexts.
彼はキリスト教の不信者だ。
He is an unbeliever in Christianity.
不信者にも分かりやすい説明が必要だ。
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.