Joel was written during the locust plague, a time of great distress for the people. The prophet sees in the devastation of the locusts an indication of the coming of the day of the Lord. Therefore all must repent with fasting and mourning. With repentance, however, there is a promise of relief and God’s blessing for Israel.

Joel 2

2:27 And ye shall know that I am in the midst of Israel, and that I am the LORD your God, and none else: and my people shall never be ashamed.