Hardly could they fight against others, when their own convictions had started to wear thin.

wear 在这里是动词，意思是"磨损、消耗"，引申为"逐渐减弱"。 thin 是形容词，意思是"薄的、稀薄的"，在这里形容信念变得"薄弱、不坚定"。 wear thin 这个短语表示"逐渐变弱、变得不再有效"，用来形容信念或耐心等抽象事物逐渐减弱的状态。