確信
かくしん · kakushin · JLPT N1 · noun
conviction
確 = certain + 信 = believe/trust, so 確信 = certain belief = conviction
確 means firm or sure, and 信 means trust or faith. Kakushin is trust that has become firm, a deep confidence in what you hold to be true.
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.