Albert Barnes on 1 Thessalonians 5:4

But ye, brethren, are not in darkness, that that day should overtake you as a thief. The allusion here is to the manner in which a thief or robber accomplishes his purpose, He comes in the night, when men are asleep. So, says the apostle, the Lord will come to the wicked. They are like those who are asleep when the thief comes upon them. But it is not so with Christians. They are, in relation to the coming of the day of the Lord, as men are who are awake when the robber comes. They could see his approach, and could prepare for it, so that it would not take them by surprise.

(c) "brethren" Eph 5:8, 1Jn 2:8