You should continue more and more to behave in the manner that you received from us as the appropriate way of behaving and of pleasing God. (4:1)
Paul only had a short time with the Thessalonians before he was chased out-of-town. Still, he had discussed how they should behave as Christians. For Paul, ethics were fundamental to the way of Christ.
In this chapter’s discussion of basic ethics and beliefs, it is interesting what Paul discusses: sexuality, money, and death.
This is God’s will, you see: he wants you to be holy, to keep well away from fornication. Each of you should know how to control your own body in holiness and honor, not in the madness of lust like Gentiles who don’t know God. (4:3-5)
Now, about charitable concern for the whole family: I don’t really need to write to you, because you yourselves have been taught by God to show loving care for one another. . . . Work with your hands, just as we commanded you, so that you may behave in a way which outsiders will respect, and so that none of you may be in financial difficulties. (4:9, 11b-12)
Now concerning those who have fallen asleep . . . We don’t want you to have the kind of grief that other people do, people who don’t have any hope. (4:13)
Think about it: aren’t inappropriate thinking and behaviors related to sexuality, money, and death especially dangerous? Each can significantly alter the course of one’s life. A life lived in immorality and licentiousness degrades and endangers others and oneself. Greed makes the turning of a buck the most important goal and people who stand in the way a target for removal. Laziness is contagious and makes many other vices necessary. Unchristian thinking about death may be the least obvious, but consider how life is lived when one believes the grave is the end. There is also a common element in these three: each makes one live in the here and now with no gratification delayed and no thought to the future.
One more thought: is western society not obsessed with sex, money, and a terminal view of death? How important it still is for us to believe that contrasting views about these three topics must be fundamental teachings for young Christians.