A locust invasion precipitated the vision of the next book in our yearlong study of the Old Testament writing prophets. We refer to the book of Joel, which describes a plague of locusts under the old covenant as a type of the day of the Lord (Joel 1:4, 15–16). Following the invasion of locusts at one point in the history of God’s people, the Lord sent Joel to warn them that a worse day of judgment was yet ahead for the impenitent. Of all the writing prophets, Joel is perhaps the hardest to date. The first issue is that we know nothing about Joel except that he was a prophet and the son of Pethuel (1:1). Although Joel is not an uncommon name in the Old Testament, appearing frequently in the genealogies of 1 Chronicles (for example, 4:35; 5:8; 6:33) and in Ezra 10:43, Nehemiah 11:9, and elsewhere, we cannot with certainty identify the prophet with any of these individuals. The name of Joel’s father, Pethuel, appears only in Joel 1:1. The second difficulty is that in his book, Joel himself offers few clues as to when he lived. Scholars have dated the book anywhere from the ninth to the second century BC. Most settle for a date just prior to the Babylonian exile or the immediate post-exilic period. We are dating the book just before the exile, though the book’s date bears little on its interpretation. Joel took up his pen in response to the disaster of a locust invasion (v. 4). Ancient people often faced locust swarms, and even today one swarm can wreak incredible devastation, with up to 120 million locusts covering thousands of square miles. Modern technology can barely contain locust swarms, so we can only imagine their horror in the ancient world. Locusts could virtually eradicate a nation’s food supply, resulting in rising food prices, subsistence living, and disease, as well as economic catastrophe when no food was left for trade. Joel describes many of these effects (vv. 7–12). Like the rest of the prophets, who speak of the day of the Lord as both a present occurrence and a future event in which God issues His final judgment on evil, Joel points to the locust invasion as a day of the Lord that foreshadows a greater day to come (vv. 13–18; see Ezek. 7:19; Obad. 15; Mal. 4:1–5). Given the locust devastation that Joel describes, the final day of the Lord must be far worse for the impenitent. The New Testament gives us the fullest picture of this day, when those who hate God will be cast into hell—where the “worm does not die and the fire is not quenched” (Mark 9:42–48).