Joel 2 begins with a prediction of an invasion of Israel by a powerful army. The enemy is not like any enemy they have encountered before, indeed there will not even be such a powerful foe in the future (v. 2). They destroy everything (v. 3), natural barriers are no obstacles to their progress (vv. 4-5), they do not get tired (vv. 6-7), and there is no effective resistance against them (vv. 8-9). This is an army that uses more than human resources. What is surprising is that the Lord is the One who is in command of it (v. 11). The lesson from this description is that the Lord can sometimes use previously unheard of weapons against his people when they fail to obey him.
Yet through the prophet the Lord also sends a call for repentance to them (vv. 12ff.). The repentance he is looking for is internal, not merely external. When such repentance takes place, the penitents will be fasting (because they have no desire to be distracted from seeking God, even by legitimate things) as well as weeping and mourning (dry eyes are signs of unaffected hearts).
Why should they return to the Lord? The basic answer is that he is their covenant God who does not wish to bring judgement on them. He wishes to deal with them in gracious and merciful ways; therefore he delays his judgements. If they repent, then he will remove the threatened judgement that the prophet had just described.
Joel urges them to take this possibility seriously (v. 14). They could still know God’s blessing on their harvests, although the prophet highlights that the best part of such a blessing is having something to give to God.
The prophet urges them to call a national gathering of repentance (vv. 15-16). All ages (old and young, including infants) are to be present, and even those newly married are to take part. Those age groups highlight the priority that the prophet wants people to have. The leaders of God’s people are to pray for deliverance, and pray for it with tears, so that the unbelieving nations would not deride the Lord’s cause (v. 17).
This passage is both a rebuke and an example. It is a rebuke of our indifference to the spiritual decline all around us (how many tears were shed today for the spiritual state of our country?). It is an example of how to respond when we want the Lord to reverse the decline.