Jerome : (Verse 31.) And your strength will be like the ash of the broom. For ash, ἀποτίναγμα is interpreted by Symmachus: when the broom is beaten, and whatever dirt it has is thrown away. So all the strength and pride of the sinners and evildoers of Israel, who have forsaken the Lord, and therefore have been consumed: and they have sacrificed to idols, and are ashamed in the gardens, which they have chosen; they will be reduced to the refuse of the broom, which is consumed by a light fire. For it follows: And her work, that is, your strength, or idolatry, in which you have erred, will be consumed by a small spark. And both will be kindled together: and there will be no one to extinguish it. And surely the knowledge of the Jews, and all the works they do, whether it be idolatry or Jerusalem, in which idolatry was found: and when the Lord kindles it, no one will be able to extinguish it. All these things we can understand about conflicting teachings: that both teachers and disciples will perish together, and all their works will be fuel for the fire. — Commentary on Isaiah