Joel 2:21

Fear not, O land; be glad and rejoice: for the LORD will do great things.

The promise is addressed to those who heed the warning of judgments and calamities coming upon those who continue to dishonor God, and in that setting anyone who will may escape by fleeing from the wrath to come. It comes after blowing the trumpet in Zion, sanctifying a fast, and calling a solemn assembly of the people with elders, children, bridegroom and bride. In that assembly the priests weep between the porch and the altar, pleading for God to spare His people and not give His heritage to reproach and to rule by the heathen.

Then the Lord is jealous for His land and pities His people, sending corn and wine and oil for their satisfaction and removing their reproach among the heathen. The land is therefore commanded to fear not but to be glad and rejoice, because the Lord will do great things and act powerfully. That joy follows God's pity for His people and His restoration of what calamity destroyed, bringing a fruitful land, abundance, and an end to shame.

The great things include springing pastures for the beasts of the field and fruit-bearing trees, with fig tree and vine yielding their strength. They include the former rain moderately, with former and latter rain together, floors full of wheat and vats overflowing with wine and oil. They include restoration of the years eaten by locust, cankerworm, caterpillar and palmerworm, eating in plenty with satisfaction, praise to the name of the Lord who dealt wondrously, and His people never ashamed. The chapter gives authority to preach the warning that the hour of God's judgment has come. The Lord's doing great things includes sending His Spirit upon messengers with such power that they cannot resist speaking, even young laymen without office or permission from church or state.