Jamieson, Fausset and Brown on 1 Thessalonians 5:2

2. as a thief in the night--The apostles in this image follow the parable of their Lord, expressing how the Lord's coming shall take men by surprise (Mt 24:43; 2Pe 3:10). "The night is wherever there is quiet unconcern" [Bengel]. "At midnight" (perhaps figurative: to some parts of the earth it will be literal night), Mt 25:6. The thief not only gives no notice of his approach but takes all precaution to prevent the household knowing of it. So the Lord (Re 16:15). Signs will precede the coming, to confirm the patient hope of the watchful believer; but the coming itself shall be sudden at last (Mt 24:32-36; Lu 21:25-32, 35).