Jerome : (Verses 15-16) I have made you like a new threshing sledge, with sharp teeth. You will crush the mountains and reduce them to dust; you will make them like fine dust. You will scatter them to the wind, and a whirlwind will blow them away. Then you will rejoice in the Lord; you will glory in the Holy One of Israel. LXX: Behold, I have made you like new grinding wheels of a wagon, in the manner of saws: and you will grind the mountains and crush the hills: and you will scatter them like dust and the wind will carry them away, and the storm will disperse them. But you will rejoice in the Lord and in the holy ones of Israel. And what follows: I have set thee as a new threshing cart, having tearing beaks, signifies that the preaching of the Gospel crushes the opposing powers and lifts up the spiritual wickedness that exalts itself against the knowledge of God; which, according to the variety of pride, are called mountains and hills. And when these are taken away and scattered like a whirlwind, let Israel exult and rejoice in the Holy Lord of Israel. We can also say that an ecclesiastical man, having the serpents of evangelical preaching on a new wagon, which does not work at all in the oldness of the letter, but in the freshness of the spirit, crushes the hardest hearts of unbelievers: separating the wheat from the chaff, and breaking mountains and hills, that is, all the leaders of heretics, who, being crushed and broken, are reduced to dust and taught that they are nothing, so that those who were gathered wickedly may be divided into their own good. — Commentary on Isaiah