John Calvin 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)

The Prophet continues with the same subject: God will eventually become an avenger of the wrongs of His people when they are unjustly harassed by profane men. We indeed know that God does not immediately help His servants but waits, as if He did not regard their troubles. However, He does this to test their patience. Then, at the appropriate time, He declares that He had not been indifferent, but had noticed the evils done to them and had postponed punishment until the wickedness of His enemies was complete.

So he says now that God will eventually be the defender of His people against all the nations assembled from every direction in the Valley of Jehoshaphat. We have already said enough about this valley. But the main point is that the afflictions of the Church will not go unpunished, for at the right time God will ascend His tribunal and cause all nations from every part of the earth to assemble and be judged there.