信者
しんじゃ · shinja · JLPT N1 · noun
believer
信 — person 亻 + speech 言
信 combines 亻(person) with 言(speech), depicting a person standing by their words. The original sense is 'truthfulness' or 'sincerity'—what a person says can be trusted. From this it extended to 'believe,' 'trust,' and later 'message' or 'letter,' since a letter carries one's word.
者 — old/elder 耂 + say 曰
Originally depicted sugarcane or a similar plant, but the form was reanalyzed as 耂 (old) over 曰 (say). It became a grammatical suffix meaning 'one who…' or 'the…thing,' nominalizing verbs and adjectives. Today 者 mainly marks agents or categories, as in 学者 'scholar' or 记者 'reporter.'