Joel 2:21 - Matthew Poole's English Annotations on the Holy Bible

Fear not, O land. In Joel 2:10, the land is said to quake at the approach of this devouring army; now at the approach of mercy it is encouraged, and commanded not to fear; by the same figure and in the same sense, it should not fear now as before it did quake. Be glad and rejoice: this tells us that fear not was a meosis; shout for joy, express the greatest confidence. For the Lord, who is your God whom you seek, who by me assureth you it is his gracious purpose to do great things for your good, will do great things: either this explains that last clause, Joel 2:20, or is antitheton to it; the locusts, God's messengers and servants. did great things against, but now God will recall these, and do great things for Israel.