This verse starts with declares the LORD a phrase often used for divine messages in the prophets. It is the only time it appears in Joel. This shows that the invitation came directly from God.
return to Me with all your heart: The Lord urged his people to repent because judgment day was near. They still had a chance to avoid destruction by truly returning to the Lord their God.
Unlike other prophets, Joel does not list the sins of Judah and Jerusalem. Perhaps they were so obvious that he did not need to mention them.