Theology: The Book of Joel gives us the theology concerning the “day of YHWH.” This is a day that God decides to bring judgment upon His people. In Joel, that judgment takes the form of extreme famine, where locusts have devoured all of the crops. This judgment is taking place because the people have sinned against God, and are, therefore, no longer under His protection. In the ancient Near East, the people are only under the protection of deity when they remain in his or her favor. As long as they have the god’s favor, the elements of chaos (e.g., invading armies, natural disasters, etc.) are held at bay. The protection of the deity functions much like a damn holding back mountains of water. When that favor is lost, the damn is broken, and the waters of chaos flood in and destroy everything. The day of YHWH is the day He decides to judge His people for their sins and no longer hold back chaos.
Ethics: The response to this coming judgment, as in all of the prophets, is to repent. Repentance is said to look like a complete focus of thoughts geared toward obedience to God, fasting (i.e., the sacrifice of food/resources in order to take care of the poor among God’s people), weeping and mourning (i.e., the external emotional signs that accompany repentance. However, a caution is given to not confuse an emotional sorrow with real repentance. Hence, Joel urges the people to “rend your mind and not your garments.” This is defined as “returning to YHWH your God” (2:13).
The people have hope in this because God is “gracious and compassionate, slow to anger, full of hesed, redirecting His favor away from chaos” (v. 14); but it is not a sure thing that God will, in fact, turn His judgment back (v. 15). This is a common theme in the prophets, and it conveys the idea that God is not mocked. He may or may not forgive. He has the discretion to do so or to refrain from doing so. No one, and no act (even an act of repentance), controls Him. However, because of His character, there is hope that He will turn from His wrath and bless the people instead. The hope of repentance is that God will remove the threat of chaos and restore the people and the land to prosperity.
Repentance is also corporate, not just individual (2:16), and corporate repentance is to be led by the priests, who are to intercede for the people (v. 17).
Joel is really a warning to Israel to repent before the day of the Lord, i.e., before God withdraws His protection and lets chaos flood in. In this regard, Peter’s use of Joel in the Book of Acts is consistent with its theme. He warns them of the oncoming judgment for their killing of the Messiah. They, however, do not repent, and God withdraws His protection. The chaos that ensues when the Romans attack the city is, therefore, a result of God withdrawing this protection on the “day of the Lord,” precisely, because it functions the same way as it does in Joel. It is not because there is only one day of the Lord, but because the day of the Lord is any day that God withdraws His protection from upon His people and lets chaos come bursting in.
Joel also connects the day of the Lord judgment of locusts to one or more in the future, where the people of God repent and are, therefore, given the Spirit of God so that revelation becomes abundant. This is the opposite of the judgment of God that withholds revelation from people. Instead, even the children will prophesy, and all who call upon the name of the Lord will be placed on the Lord’s holy mountain/Jerusalem and will escape the judgment, and all of their enemies (i.e., agents of chaos) will be judged instead.