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"，在这里表示"变得薄弱"。 wear thin 这个短语字面意思是"磨损变薄"，比喻指"逐渐减弱、变得不可靠"，常用来形容信念、耐心等抽象事物的减弱。