5:2 day of the Lord. This is the first written mention of “the day of the Lord” in the New Testament, although Peter had quoted the phrase from the Old Testament (Joel 2:31) in his sermon on the day of Pentecost (Acts 2:20). This phrase is used over thirty times in the Old Testament. In context here, it refers to the coming of the final period of God’s judgment on the earth.
5:2 thief. Although the day of the Lord would be a period of great judgment on God’s enemies, it would begin unexpectedly and quietly. The Lord Jesus had also used the figure of the thief coming secretly, and the Thessalonians already knew this from Paul’s teachings (see Matthew 24:42-44; also note II Peter 3:10; Revelation 3:3; 16:15). Therefore, we should be constantly watchful for the Lord, not for various signs preceding His coming. Paul was watching throughout his life (II Timothy 4:8), and here he urges: “let us watch and be sober” (I Thessalonians 5:6).
5:3 Peace and safety. “Safety” means “security.” The day of the Lord will burst suddenly on an unsuspecting world at a time when there is great worldwide concern with peace and security, possibly when such a condition is finally achieved by a great charismatic world leader. This is the development being planned and vigorously promoted today by the many New Age cults and movements seeking a new world order. If this is a sign, however, it applies to the day of the Lord, not to the rapture, which could occur at any time and, in principle, could have occurred at any time in the past, even during the apostolic period. Otherwise, there would be no point to Paul’s exhortation to watch for Christ’s return. Note the contrast between “they” and “we.”
5:3 as travail upon a woman. The day of the Lord is as unexpected as a thief, but once it begins, its destructive consummation is as inevitable as birth after travail.