The passage
      v1-2. Paul encourages the believers in Thessalonica to apply themselves "more and more" to their Christian "walk", in line with the instructions he gave them when he first ministered to them. Instead of serving their own interests, they should seek to "please God", or more rightly, serve God, as in fact they have been doing. The Greek word translated "please" would be better rendered "serve". In the end, the only act that is pleasing to God is repentance and faith. Trusting Christ pleases God far more than our compromised behavior.
      v3. To the modern mind it seems strange that sexual immorality should be first and foremost in Paul's thinking, but both the theological importance of the one flesh union of marriage, along with the sexual laxity of Greek society, gave it a high priority for him. God's intention is that his children "should be sanctified", ie. through the power of the indwelling Spirit of Christ they should become what they are already in Christ. This intention is easily stifled when we give ourselves over to personal corruption. "Sexual immorality" here means illicit sexual intercourse.
      v4. From the negative, abstain form impurity, Paul gives the positive, keep the body pure. This verse is not easily translated and may actually be an exhortation to "control" (in the sense of "possess") their own "body" ("vessel", ie. their wife). "Live continently" is most likely the substance of the exhortation. Such is an appropriate expression toward sanctification, ie. "is holy", and is also "honorable".
      v5. The God-empowered person is to rule their body rather than be caught up in lustful passions (overmastering desire rather than aggressive desire). Such behavior is typical of the non Jewish world (here Paul means unbelieving Gentiles). They do not know God and therefore tend toward animal behavior.
      v6. Sexual immorality wrongs the third party, either the person's married partner or future partner. "The Lord will punish" such behavior. Immorality has consequences both here and in the day of judgement, and these consequences should not be ignored. God's righteous judgement exacts a terrible cost. The believer cannot expect to sidestep these costs in the here and now, although thankfully sin's eternal cost is carried by Christ on the cross.
      v7. God's "call", in the sense of intention, is not to free us to indulge in impure or immoral sexual acts, but rather to move us into the sphere where God's sanctification takes place, that our life may be increasingly aligned to the character of Christ through the power of the indwelling Spirit. God certainly does not intend for believers to live "impure" (unclean) lives.
      v8. The believer who thinks they can get into sexual sins with impunity needs to understand that their behavior is nothing less than treating God with indifference. Above all, it is an affront to the Lord. In a sense, it is a sin against the Holy Spirit. We carry with us the very presence of the Lord in the gift of the Holy Spirit. We treat as nothing his presence when we act willfully.

Sexual immorality
      Martin Luther once wrote, "a Christian man is a perfectly free lord of all, subject to none. A Christian man is a perfectly dutiful servant of all, subject to all." Freedom and service, the two go together.
      For a male, and increasingly within our society for a female, sexual immorality is a constant temptation. Greek society of the first century virtually encouraged sexual immorality. It was "excused by parents, commended by moralists, and consecrated by religion", Wordsworth. "Almost every form of sensual indulgence beyond the limits of marriage was permitted", Lecky. Today, affluent Western societies parallel Greek society of the first century and so believers need the same warning that Paul gave the Thessalonians all those years ago. "Avoid sexual immorality", "control your own body."
      We all of us chase happiness, we all want to be fulfilled, possess identity, meaning, and somehow sex seems to provide the answer. There is something eternal in sexual union that gives us a taste of immortality. We possess the other and seemingly possess their life. Yet, it is but a taste, an image of an eternal union possible only with God.
      The apostle gives three reasons why we should flee from sexual immorality and aim at chastity and marital fidelity:
        i] God will act against illicit sex. It is true to say that God has designed the world so that sexual immorality brings its own disastrous consequences. We can't sow the wind without reaping the whirlwind.
        ii] Sexual immorality affronts the whole of our Christian life. The path of sanctification is the way of happiness, not immorality.
        iii] Sexual immorality is a sin against the Holy Spirit. It personally affronts God.
      Of course, we have all fallen short of God's ideal of purity in some way or other. We may well remember a misspent youth, or an intimate relationships that has come apart and now replaced, or even the machinations of the mind. Without in any way undermining God's ideal of purity, we may need to be reminded again that He is a merciful God.

v2
      oidate (oida) perf. "you know" - Repeating the point made in v1.
      dia tou kuriou Ihsou "by the authority of the Lord Jesus" - through the Lord Jesus. "By the authority" is a bit of a stab, but is also adopted by Moffatt, Weymouth, Williams... A literal rendering, such as NAB, is safer.

v3
      gar "-" - for. Untranslated, since this conjunction often only indicates another step in the argument rather than a logical conclusion.
      qelhma (a atoV) "will" - will [of God]. Lightfoot suggests that God's will is not just that we "be sanctified", but extends to avoiding sexual immorality, honouring a wife, caring for a fellow believer... This is indicated by the lack of an article. God's will is our holiness, our Christ-likeness. "God wants ...", TEV.
      oJ aJgiasmoV (oV) "sanctified" - holy. God wants our being to be as Christ is.
      apecesqai (apecw) pres. inf. "should avoid" - to abstain, be far off from. Is this infinitive in apposition to "sanctification", which is itself in apposition to "the will of God"? This seems unlikely, given that sexual morality is but one aspect of Christ-likeness. Probably epexegetic, as NIV. "Completely free from", TEV.
      thV porneiaV (a) "sexual immorality" - Words such as "fornication" and "unchastity", are archaic, "sexual relations outside marriage" probably makes the point for the modern mind. "No one should have sexual intercourse with anyone except their own marriage partner" is a possible paraphrase, although somewhat legalistic. It is possible that Paul is referring to sexual union within the degrees of consanguinity and affinity, an issue raised at the Jerusalem conference, Acts 15, but he is more likely referring to sexual laxity and asking the believers to aim above the low standards evident in pagan society. Sexual morality is not, in itself, holiness, but for the New Testament writers, is a very expressive element of our sanctification. The imagery of Hosea, of Israel's flirtation with other God's and the way marriage is linked to union with Christ, to oneness with the divine, along with the total sexual laxity of pagan society, has propelled sexual immorality to the top of the evil list. Is it possible that the "greed is good" philosophy of modern business long ago propelled itself to the top of the list, but the church, infested with the philosophy of "success", has failed to recognize this? Of course, such a question raises the issue of Biblical interpretation, and in any case, may fail to give due weight to the "one body" doctrine of the scriptures.

v4
      eidenai (oida) inf. "that [each of you] should learn" - to know, learn. The infinitive is probably epexegetic, see v3. The issue here is not so much learning, but doing. The purpose of the exhortation is that the Thessalonians "control" their behavior; "live continently". This has prompted the simple translation "respect and honour your wives", CEV.
      skeuoV (oV) "body" - vessel, object. The meaning is unclear which is why translators divide: i] "control his body keeping it pure and treating it with respect", Phillips; ii] "procure himself a wife in purity and honour", Weymouth.
      en aJgiasmw/ kai tim/ "that is holy and honorable" - in sanctification and honor/respect/esteem. "In a holy and honorable [way]", TEV.

v5
      paqei (oV) "passionate" - emotion, experience ...... lust, passion. "Passions and desires", Barclay; or a noun functioning as an adjective, "sensual passions", Moffatt.
      kaqaper comp. "life" - just as, like, as. The Gentiles know no better, but believers do.

v6
      tuJperbainein (uperbainw) inf. "[no one] should wrong" - to overstep, cross a boundary. The infinitive, as with "to take advantage of", is still probably epexegetic what is Gods will, v3. Here probably in the sense of intruding on an existing marital relationship, although some commentators think that Paul is now speaking about financial dealings with a brother, "no one should try to overreach his fellow man in business", Barclay.
      pleonektein (pleonektew) inf. "take advantage of" - to wrong. As noted above, the sense "greedy" is adopted by some commentators.
      ekdikoV (oV) "[the Lord will] punish" - an avenger/avenging [lord]. A sense of "just punishment" is better than "vengeance." Also used to describe the function of civil magistrates, Rom.13:4. "He (Jesus/God) punishes everyone who does such things", CEV.

v7
      epi ..... en "to [be pure, but] to [live a holy life]" - [God did not call us] to [impurity, but] in [sanctification]. God did not call us to/for impurity (prep. + dat. = purpose, "not called for the purpose of indulging in impure or immoral sexual acts", Wanamaker), but in sanctification ("into the sphere where God's sanctification takes place", Wanamaker). Note the sense of the two prepositions. "God has not called us to uncleanness, but his call is addressed to us in our state of sanctification", Turner (en, "in" = "within", "in the sphere of").