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"，指信念逐渐减弱的过程。 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"逐渐减弱、变得薄弱"，用来描述信念或耐心等抽象事物的消耗过程。