Like other Minor Prophets, Joel is a book calling for repentance. Just because he is writing to God’s chosen people does not mean that they will necessariliy be spared from judgment. They have sinned against God and have been unfaithful to him. Joel preaches that their restoration will come once they turn from sin, and turn to God.
It’s not always a great job to be a prophet. Joel (and others) have to deliver messages against their people. They are messengers of bad news. They announce the sin of their peers. This is a quick way to make enemies, then and now.
But their message is not a despairing one. Look for the words of hope that are held out for God’s people. In the end, despite their lack of faith, God will renew them and will make his residence among us once more (Joel 3:17).