Charles Ellicott Commentary 1 Thessalonians 5:4

"But ye, brethren, are not in darkness, that that day should overtake you as a thief:" — 1 Thessalonians 5:4 (ASV)

But you.—“Though the world (which lies in darkness) may be surprised at the coming of the Day, you, members of the Church, living in the light, cannot be surprised.” The words “in darkness” seem to be suggested by the mention of “night” in 1 Thessalonians 5:2; and the chief thought (as the succeeding verses show) is that of lethargy, inattention, torpor, not so much either ignorance or sin.

That day.—Literally, the day: so that it does not mean the Judgment Day simply as a point of time, but brings out its characteristic of being a day indeed. (Compare to 1 Corinthians 3:13.)

As a thief.—There is another reading which has two of the best MSS. and the Coptic version in its favour, and the judgment of Lachmann and Dr. Lightfoot, “As thieves.” But not only is the evidence from the MSS. strongly in favour of the Received text, but the whole context shows that St. Paul was not thinking of the day as catching them at evil practices, but as catching them in inadvertence.