The day of the Lord is mentioned in the Bible over twenty times, and refers to a day at some future but unknown time, when the people of the earth will meet God’s judgment. For those who are in opposition to Him, who deny any need for His mercy and grace, that day will be a terrible day of battle (Isaiah 2:12; Isaiah 13:6; Ezekiel 30:3; Revelation 16:14), which they are doomed to lose. In the Bible, the day of the Lord is almost exclusively painted as a day of destruction, darkness, and division, during which even the earth itself will suffer (Isaiah 13:6; Ezekiel 30:3; Joel 1:15; Joel 2:1, 11; Zephaniah 1:14-15; Zechariah 14:1; 2 Peter 3:10).
The prophet Joel describes the day of the Lord as a day when destruction will come from the Almighty. He shows God in His camp, with an army of such power that none can hope to defeat it. Yet, Joel also mentions multitudes gathering against God in a place called “the valley of decision” where this battle is to take place (Joel 3:14). This is likely describing the battle of Armageddon, when all the nations of the world will be driven by Satan to fruitlessly come against God in battle (Revelation 16:13-16).
The New Testament also calls the day of the Lord a day of wrath, a day of visitation, and the great day of God the Almighty (Revelation 16:14). It is said that the day of the Lord will come suddenly, like a thief in the night (1 Thessalonians 5:2; 2 Peter 3:10). Christians are told to be prepared for it, though it will not be a time of judgment for believers. The pride of those who reject God will be humbled on that day (Isaiah 2:17). But anyone who is covered by Christ’s work on the cross will not have to face judgment. This was Jesus’ statement about why He came to earth: “For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God” (John 3:17-18). When the day of the Lord comes, it will be fearful for those who have not believed in the Son of God.