Brief review of setting. Paul continues on the topic he began in 4:13-18 – the implications of Jesus' return. In that passage, Paul informed them of knowledge they didn't have (4:3) – that when Jesus returns He will permanently reunite living and dead Christians. In this passage, he reminds them of the facts they already knew about Christ's return, and explains how to live in light of it (read 5:1-11).
"The day of the Lord" (5:1) is a biblical term referring to God's intervention to judge and rescue. The ultimate "day of the Lord" refers to a complex of events culminating in the personal, bodily return of Jesus to this earth as its rightful Messiah, or Ruler.
It predicted the Messiah's death – when He would die (Dan. 9), how He would die (Ps. 22), and why He would die (Isa. 53). Jesus predicted His own death as the fulfillment of these predictions (with no good reason to fulfill them this if He was not the Messiah).
It predicted the Messiah would be resurrected from the dead, and Jesus applied these predictions to Himself. This is perhaps the best attested event in ancient history.
Jesus also predicted a long period between His first and second Comings during which the good news of His death for our sins would be spread through His followers to every ethnic group in the world (Matt. 24:14). The substantial fulfillment of this amazing prediction is the best-kept secret in our world.
Since the Bible has been so accurate on its past predictions about Jesus, we have excellent reason to trust it when it predicts His future return. Contrast this to blind faith secular hopes in technology, extra-terrestrials, human goodness, etc.
Many Christians affirm this event, but warn against making too big of a deal about this event. But it is the climactic event of salvation history that is foretold throughout the entire Bible. The first prediction comes immediately after the Fall (Gen. 3:15). Every Old Testament prophet speaks of this event. Entire Psalms were composed to be sung at His Coming. Jesus speaks of it in every one of His discourses, and focuses entirely on it in one of them. Almost every New Testament letter speaks of it; many (like 1 Thess.) bristle with references (28 references/30%). The final book of the Bible (Revelation) focuses on Jesus' return. The whole plot of the Bible depends on this!
The day of the Lord will come. The question is: Will it be a good day or a bad day for you? It depends entirely on whether or not you belong to Him.
For those who don't belong to Jesus, His return will be a bad surprise (re-read 5:2,3; notice the pronouns). They will expect human history to keep going like it has, but it won't – to their great and irreversible loss.
It will be "like a thief in the night" – you go to bed thinking your belongings are secure, but when you awake you find that someone has stolen them and you can't get them back (EXAMPLE)! In the same way, people who don't belong to Jesus will be caught off guard by His return. They will think that they are safe and secure because of their investments or social prestige or military might, but Jesus' return will destroy this security and bring them great loss. It will be "like labor pains upon a woman with child" – sudden and severe, and once they start they keep going until the child is born. In the same way, the judgment brought by Jesus will be sudden, and there will be no escape from it.
But if you belong to Jesus, His return is a great day (re-read 5:4,5; notice pronouns). It's not something that will take us by surprise; we expect it. It's not a nightmare we want to avoid thinking about; it is great day we look forward to ("Maranatha!").
Re-read 5:9,10. It's a great day because then we get to be reunited with believing loved ones (LAST WEEK), and best of all, we get to be with Him (and become like Him – see 1 Jn. 3:2; Col. 3:4).
We don't have to worry about judgment because He died for us! He already bore the judgment we deserve for our sins against God, and all we had to do is humbly receive this as His gift. The moment we received His death for us, we became destined for salvation rather than wrath (see also Jn. 5:24).
Real life is full of no-do-over decisions (EXAMPLE). The biggest one of all is your decision about Jesus. It will have a ripple-effect that not only changes this life, but also overflows into all eternity. Why not join us in admitting that you need His death for your sins? Why not entrust yourself to Him, and join us in looking forward to His return?
The responses to Jesus' return
How should Christians live in response to this fact of Jesus' return? Paul anticipates this question and gives two answers to it.
Re-read 5:6-8a – we should be "alert and sober." Paul is repeating what Jesus said when He predicted His return (Matt. 24:35,36,42): "I am definitely returning, you don't know when I am returning, so you should be ready at all times." Let's consider what this means and how to cultivate it.
These words mean to be wakeful, watchful, to have your wits about you, to be ready. An alert and sober Christian lives each day remembering this all-important fact that Jesus is returning. The opposite of alert and sober, of course, is to be asleep and/or drunk. The problem with people who are drunk or asleep when they shouldn't be is that they're unaware of things they should be aware of, so they're a danger to themselves and others (e.g. DRIVING DRUNK OR WHILE TEXTING). Paul is using these terms figuratively to describe true Christians who live in a way that is practically oblivious to Jesus' return.
What does it look like as a Christian to be "alert and sober" versus "asleep/drunk?" The Bible fleshes out many contrasts between these two mind-sets. Here are a few: