Isaiah’s threshing sledge
Mountains are to be threshed, in a prophecy of Isaiah. The saints are the “threshing sledge.”
Isaiah 41:15-16 NIV
“See, I will make you into a threshing sledge,
new and sharp, with many teeth.
You will thresh the mountains and crush them,
and reduce the hills to chaff.
You will winnow them, the wind will pick them up,
and a gale will blow them away.
But you will rejoice in the LORD
and glory in the Holy One of Israel.”
The International Standard Bible Encyclopedia states that ancient threshing floors were located in exposed, windy places, as wind was required for winnowing. 
Threshing.– The threshing-floors are constructed in the fields, preferably in an exposed position in order to get the full benefit of the winds. If there is a danger of marauders they are clustered together close to the village. The floor is a level, circular area 25 to 40 ft. (7½ to 12 m.) in diameter, prepared by first picking out the stones, and then tamping or coating it with marly clay. A border of stones keep in the grain. The normal method of threshing seems to have been by the trampling effect of oxen or donkey hoofs as these animals were driven around the floor. (Dt. 25:4), or by the use of a drag, the bottom of which was studded (2 S. 24:22). The supply of unthreshed grain was kept in the center of the floor and fed into the path of the animals. Constant turning of the partly threshed grain hastened the process of breaking all the stalks into short pieces and tearing off the husks. This mixture of chaff and grain was then winnowed by tossing it into the air so that the wind may blow away the chaff. When the chaff was gone, the grain was tossed in a wooden tray to separate from it the stones and lumps of soil which clung to the roots when the grain was reaped. The difference in weight between the stones and grain makes separation by this process possible. The grain was then stored in the common store jar, usually for home use.
P. C. Anderson wrote about the sounds associated with threshing: 
When a threshing sledge is pulled on the threshing floor by one or two donkeys, oxen, horses or mules in a circular trajectory at a constant low speed over a mattress of sheaves of grain, the plod of animal feet, often sounds of chains or bells are accompanied by a continuous sound of cracking and swooshing which corresponds to the pulling and smooth working of the instrument as it sets the plant material in motion in such as a way as to rapidly thresh the grain and cut up the straw into tiny pieces.
A key question about Isaiah’s prophecy is, what do the mountains represent? Many commentators claim they represent the governments of nations, which were Israel’s enemies, but the threshing metaphor implies some kind of separation operation is performed on them.
The idea of threshing mountains implies there is something within them that needs to be extracted, and separated out, like grains of barley, oats, rye, and wheat from straw.
In the Old Testament, and in Isaiah’s prophecy, mountains and hills are metaphors; they represent revelations of God; covenants, promises, and prophecies. This is evident in Genesis, in the blessing of Joseph by his father Jacob. He said the blessings that he inherited extended “unto the utmost bound of the everlasting hills.” [Genesis 49:26] This alludes to their height, and thus their heavenly origin, and their eternal duration.
Prophecies in the Old Testament relate to the covenant, and the land promise to Israel was a type of the spiritual things contained in the new covenant. Mountains are both durable, and high, so they represent things that are eternal, having lofty or spiritual meanings. Promises to the saints about spiritual things are represented by mountains.
The promised land was the location of many revelations, and mountains are the prominent parts of the promised land. Specific mountains may give their name to prophecies, such as the Olivet Discourse.
Correctly interpreting prophecies and parables extracts their true meaning. This is the threshing process, that Isaiah said is to be accomplished by the church. The chaff that is blown away by the wind consists of superficial, flawed, literal interpretations. It is the embellishment surrounding and concealing kernels of truth.
Parables are similar to prophecies; the meaning or interpretation of a parable is the grain which provides nourishment to believers.
How are mountains threshed? This has to do with interpreting the symbolic mountains of prophecy. Joel said that mountains will drop down new wine and hills will flow with milk.
So shall ye know that I am the LORD your God dwelling in Zion, my holy mountain: then shall Jerusalem be holy, and there shall no strangers pass through her any more.
And it shall come to pass in that day, that the mountains shall drop down new wine, and the hills shall flow with milk, and all the rivers of Judah shall flow with waters, and a fountain shall come forth out of the house of the LORD, and shall water the valley of Shittim.
These mountains and hills are figures representing promises of God to the saints, who seek a “better country,” which represents their spiritual inheritance. Wine and milk are metaphors interpreted in the New Testament as representing God’s word. When Jesus spoke of new wine, and old bottles, he was referring to the gospel.
And no man putteth new wine into old bottles: else the new wine doth burst the bottles, and the wine is spilled, and the bottles will be marred: but new wine must be put into new bottles.
The author of Hebrews said “milk” represents teachings intended for those who are immature: “For every one that useth milk is unskilful in the word of righteousness: for he is a babe.” [Hebrews 5:13]
The apostle Peter encouraged his readers, “As newborn babes, desire the sincere milk of the word, that ye may grow thereby.” [1 Peter 2:2]
Wheat is also a metaphor that Jesus often used to represent his teachings, for example in the parable of the sower. Harvesting of wheat involves threshing. After the stalks of grain are harvested, they are chopped and beaten or threshed, to separate the grain from the chaff.
Isaiah’s prophecy about threshing mountains alludes to this process of separating. The church is involved in beating the mountains, which are symbolic of the revelations of God, and prophecies, and the promises of God to the saints contained in scripture. Perhaps the “threshing” metaphor represents various attempts to discover the true meaning of the prophecies of scripture. In the threshing process, the stalks are broken and cut small, and the grains are rubbed against one another, and moved about, so that all the chaff can be dislodged, and blown away by the wind. The process of interpretation of prophecy is similar; various theories are proposed, which are the means of “threshing.” When these interpretations are compared one against one another, flaws are exposed in the debate, and “chaff” gets separated from the truth.
This is the role of theories of preterism and dispensationalism; these theories are opposed to each other, and are mutually destructive. Supporters of preterism expose flaws in the theories of dispensationalism, and likewise supporters of dispensationalism, when defending their views, expose the flaws of preterism. Both doctrines represent “chaff.” The grains of truth remain, to which both groups are blind.
1. Geoffrey W. Bromiley. International Standard Bible Encyclopedia. Wm. B. Eerdmans. 1995. p. 74.
Revised: 16 June 1011.
Download: Mountains in Prophecy [pdf file]
- Christ the “ensign for the nations”
- Mountains will be made low and valleys filled
- The valley of the mountains
- Some problems in Covenant Theology
- Threshing the mountains