Adam Clarke on Joel 2:13

Rend your heart - Let it not be merely a rending of your garments, but let your hearts be truly contrite. Merely external worship and hypocritical pretensions will only increase the evil, and cause God to meet you with heavier judgments.

For he is gracious - Good and benevolent in his own nature.

Merciful - Pitying and forgiving, as the effect of goodness and benevolence.

Slow to anger - He is not easily provoked to punish, because he is gracious and merciful.

Of great kindness - Exuberant goodness to all them that return to him.

And repenteth him of the evil - Is ever ready to change his purpose to destroy, when he finds the culprit willing to be saved. See the notes on Exo 34:6, Exo 34:7.