Albert Barnes Commentary Joel 3:12

"Let the nations bestir themselves, and come up to the valley of Jehoshaphat; for there will I sit to judge all the nations round about." — Joel 3:12 (ASV)

Let the pagan be awakened - This emphatic repetition of the word, “awaken,” seems intended to hint at the great awakening to Judgment, when they who sleep in the dust of the earth shall awake, being awakened from the sleep of death. Another word is used for “awakening.” On the destruction of antichrist, it is thought that the general Judgment will follow, and all who are in the graves shall hear the voice of the Son of Man and shall come forth (John 5:27–29).

They are commanded to “come up” into the valley of Jehoshaphat, since coming into the presence of the Most High God may well be called “a coming up.” For there I will sit to judge all the pagan round about, (again, literally “from round about”), from every side, all nations from all the four quarters of the world.

The words are the same as before. There “all nations from every side” were summoned to come, as they thought, to destroy God’s people and heritage. Here the real purpose is assigned, for which they were brought together, for God would sit to judge them. In their own blind will and passion they came to destroy; in God’s secret overruling Providence, they were dragged along by their passions—to be judged and to be destroyed.

So our Lord says, When the Son of Man shall come in His Glory, and all the Holy Angels with Him, then shall He sit on the throne of His Glory and before Him shall be gathered all nations (Matthew 25:31–32). Our Lord, since He uses words of Joel, seems to intend to direct our minds to the prophet’s meaning. What follows are nearly His own words;