When do people become most vicious, according to Les?
People become most vicious when they do it in the name of religion. Religious Jews, though civilized, lost all civility and became berserk because of their convictions, even gnashing their teeth at Stephen. This pattern shows religious fervor can override civility.
Les teaches that people never get more vicious than when they do it in the name of religion. In this passage, Stephen is speaking to the nation, which had received the law by the disposition of angels but did not keep it. As Stephen builds the case, God convicts their hearts. When they heard these things, they were cut to the heart and gnashed on him with their teeth. Les points out these were religious Jews, civilized people, but because of their religion they suddenly lost all their civility and became berserk because of their convictions. He emphasizes he is a literalist, so they literally gnashed on him with their teeth.