He had an innocent belief that everyone would keep their promises.

innocent 是形容词，由前缀 in-（表示"不"）和词根 nocere（拉丁语"伤害"）组成，字面意思是"不会伤害的"。引申为"天真的、单纯的"，指缺乏经验或判断力而容易轻信他人。