“But you, brethren, are not in darkness, so that this Day should overtake you as a thief“
But you, brethren,
Christians stand in stark contrast to those without Christ (the “they” of verse 3). The coming Day of the Lord will not surprise them.
are not in darkness,
Paul previously told the Thessalonians about the coming Day of the Lord so they were not in the dark about that day. God takes Christians into His confidence about future things. Non-Christians live in deluded darkness about that day.
so that this Day
Christians will not participate in the beginning of the Day of the Lord, the Tribulation (5:9-10).
should overtake you as a thief
The word “overtake” properly signifies to lay hold of; then, to lay hold of so as to possess as one’s own, to appropriate, apprehend, overtake. Jesus will have already raptured the church by the Day of the Lord (4:13-18). That is why this day will not overtake them.
“Because you have kept My command to persevere, I also will keep you from the hour of trial [the seven year Tribulation] which shall come upon the whole world, to test those who dwell on the earth” (Revelation 3:10).
God discriminates between what He does with believers and non-believers.
God always makes a distinction between His people and the non-Christian. We cannot emphasize this distinction too strongly. God will translate the Christian to heaven at the Rapture. God will leave non-Christians on earth to go into the Day of the Lord.
The children of the Light have knowledge about the coming dark day. They are in the light about God’s purpose.
The Day of the Lord is when the Lord returns to earth in great glory to judge the world. He will make things right in that day. First, He will bring great judgment on the earth during the Tribulation. Then, in His Second Coming, He will come back with the saints to establish His Kingdom on earth for 1,000 years. The Day of the Lord includes both the Tribulation and Kingdom.
“…and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, when He comes, in that Day, to be glorified in His saints and to be admired among all those who believe, because our testimony among you was believed” (2 Thessalonians 1:7-10).