unbeliever; heathen
Religious term for a person who does not believe in a particular faith, especially in Christian contexts. Can carry a slightly negative or old-fashioned nuance.
He is an unbeliever, but he is cooperative with church activities.
The missionary preached the gospel to the heathens.
異教徒 refers to a person of a different religion, often used in a broader or comparative sense, while 未信者 specifically means someone who has not yet believed in a particular faith.
Compound of 未 (mi, 'not yet') + 信 (shin, 'believe') + 者 (sha, 'person'). The exact historical derivation is uncertain, but it is a straightforward kanji compound used in religious contexts.