Isaiah 41:14-16. God tells Israel, "you worm Jacob," not to worry or fear - he will help them. Their Redeemer is the Holy one of Israel, so they need not fear anything. He will make them a threshing sledge with new sharp teeth and they shall thresh all the surrounding mountains and hills. They shall winnow these places and the wind and tempests will carry the chaff away and scatter it. They will then rejoice in the Holy One of Israel.

Here Isaiah is prophesying hope for Israel, even though he calls it a worm (!), as it struggles against its many foes. This promise that God will make them threshers of the surrounding lands can be read as a military or nation-state prophecy - they will eventually conquer these areas and their enemies. But given the frequent use of threshing as a symbol for how God sorts the good from the bad (see, for example, He Will Gather Us One by One), this can also be read to be a prophecy of how Israel, being the chosen people and the ones from whom the law and redemption will go out into the world, will actually be the means of God's threshing of the world.

The importance of the Jews in salvation history is primary - there is no other chosen people. There is a reason they were chosen - to be the foil to the world's fallen ways; to be an example to the world of the right relationship with and to the true God; and ultimately to be the line from which the Savior would come. All of this is exemplified by the image of them being a threshing sledge for the rest of the world.