信
xìn · HSK 2
letter
✉️ 信 = 人 (person) + 言 (speech); a person's word is their letter
The character combines 亻(person) on the left with 言 (speech/words) on the right. The etymology: what a person says or writes carries trust, hence both 'letter/message' and the related sense of 'trust/faith'.
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.