1-thessalonians 5:4

But ye, brethren, are not in darkness - Probably St. Paul refers to a notion that was very prevalent among the Jews, viz.: that God would judge the Gentiles in the night time, when utterly secure and careless; but he would judge the Jews in the day time, when employed in reading and performing the words of the law. The words in Midrash Tehillim, on Psalm 9:8, are the following: When the holy blessed God shall judge the Gentiles, it shall be in the night season, in which they shall be asleep in their transgressions; but when he shall judge the Israelites, it shall be in the day time, when they are occupied in the study of the law. This maxim the apostle appears to have in view in the 4th, 5th, 6th, 7th, and 8th verses. (1 Thessalonians 5:4-8)

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 people 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 people 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.

5:4 Ye are not in darkness - Sleeping secure in sin.