Paul now moves to the heart of his letter, 4:1-5:22. He begins by making the point that the following instructions are in line with the teachings he first gave the Thessalonians believers, v1-2. He then goes on to encourage them toward sexual purity, v3-8.

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, not our compromised personal righteousness.

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 Roman society, gave it a high priority. God desires 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, that we abstain form impurity, Paul gives the positive, that we 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 sexual laxity.

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, but thankfully sin's eternal cost is carried by Christ on the cross.

v7. God's "call", in the sense of invitation, is not a call to indulge in impure or immoral sexual acts, but rather a call to move into the sphere where God's sanctification takes place, a sphere where our life is increasingly aligned to the character of Christ through the power of the indwelling Spirit. God certainly does not intend a believer to live an "impure" (unclean) life.

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; a sin against the Holy Spirit. We carry with us the very presence of the Lord in the gift of the Holy Spirit and so should not willfully affront his person.

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.

Within Western society, sexual laxity is becoming the norm. 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 seem to draw to ourselves something of their being. 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 intimate relationships that have come apart and are now replaced, or a failed marriage, 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, that in Christ there is forgiveness.

1. Why is sexual immorality such a serious sin?

2. Expand on Paul's three reasons for chastity.

3. How is it that forgiveness empowers sanctification.