1 Thessalonians 5:1-9
Like us, the return of Christ was much on the minds of first-century Christians, yet Paul tells them he felt no need to write concerning its timing. Why? Because they should have known that the Day of the Lord will come like a thief in the night. There was no point in Paul trying to outline it all, as it will happen at a time that nobody can anticipate.
However, he writes something that seems contradictory in verse 4: Since they are not in darkness, that Day should not "overtake [them] as a thief." What is actually meant is that the day of God's wrath would not possess them—literally, "take them over." God's wrath would not swallow them up, or the destruction of that Day does not need to have power over them. He does not mean that it would not surprise them, but as a parallel verse clarifies, "For God has not appointed us to wrath" (verse 9), even though they will be surprised.
Verse 6 contains the same admonition seen elsewhere to be awake, to be sober, and to watch. Though we are not appointed to wrath, other verses show that we can certainly still incur it if we are not taking heed to ourselves (see Hebrews 10:26-31). So we are instructed to watch—to be vigilant about our spiritual state, to have continuous and wakeful concern over fulfilling our part of the covenant, to be on guard against spiritual dangers, spiritual drowsiness, and deception. Those who do these things, along with praying always, will be accounted worthy to escape the wrath. Simply watching down the road for a sign of the Master's return really does not prepare us for anything at all.
David C. Grabbe
'As a Thief in the Night'

The Greek word for "watch" translates better as "alert," and "sober" more correctly means "self-controlled," making Paul's advice: "Let us be alert and self-controlled." Amid the dizzying distractions of this world, one must remain alert to their appeal and disciplined enough to prioritize eternal things properly. Though difficult, the will must be forcibly set to pay attention to what matters most. Failing this, life may be conducted in darkness, eventually leading to spiritual blindness. Already around AD 50, the Thessalonians were becoming neglectful and going to sleep spiritually, much like the Parable of the Ten Virgins, where all slumbered and slept. They were awake yet asleep, awake yet not aware, perhaps ignoring overwhelming pressures and problems, hoping they would simply go away.