Paul’s first letter to the Thessalonian church made an unmistakable connection between sanctification and sexual purity. While sexual purity is only one aspect of sanctification, the Holy Spirit must have inspired Paul to draw this connection for a reason. Let’s look at the text, and then spend a little time talking about it.
Finally, then, brothers, we ask and urge you in the Lord Jesus, that as you received from us how you ought to walk and to please God, just as you are doing, that you do so more and more. 2 For you know what instructions we gave you through the Lord Jesus. 3 For this is the will of God, your sanctification: that you abstain from sexual immorality; 4 that each one of you know how to control his own body in holiness and honor, 5 not in the passion of lust like the Gentiles who do not know God; 6 that no one transgress and wrong his brother in this matter, because the Lord is an avenger in all these things, as we told you beforehand and solemnly warned you. 7 For God has not called us for impurity, but in holiness. 8 Therefore whoever disregards this, disregards not man but God, who gives his Holy Spirit to you. ~~1 Thessalonians 4:1-8 (ESV)
Notice, to begin with, verse 3. The will of God, Paul says, is for our sanctification. Sanctification is the process of developing personal holiness — of becoming more like Christ. Verse 7 elaborates on this point by explaining that God calls His people, not to impurity, but to participate in His holiness. I’ll get back to this point momentarily.
First, however, we should consider human sexuality on its own terms. Now, sexual purity, in and of itself, must be a high priority. On a strictly human level, sex outside of heterosexual marriage has so many disastrous consequences that it would take innumerable blog posts to cite them (and even then I doubt I could cover them all.) Most civilizations throughout history recognized some form of sexual ethics, indicating that people generally understand the necessity of keeping sexual expression in some sort of context.
Scripture deepens our understanding of human sexuality by teaching that God created it to reflect the relationship between Christ and the Church (Ephesians 5:22-33). As Christ desires His Bride to be exclusively devoted to Him, so He desires husbands and wives to be exclusively devoted to each other.
Furthermore, throughout the Old Testament God consistently drew parallels between Israel’s unfaithfulness to Him and illicit sexual behavior (Ezekiel 16 comes to mind). Again, human marriage between a man and a woman presents a picture of the covenant relationship between the Lord and His people. Therefore any deviation from the model of sexual expression within heterosexual marriage violates that sacred picture.
Obedience to Christ in regard to sexuality demonstrates to the world that Christians value fidelity. Our faithfulness to our spouses (even before marriage) indicates our faithfulness to Christ. We refuse to be stained by sin, preferring wholehearted devotion to our Savior.
Sexual purity isn’t the only component of sanctification, of course. But it’s an important aspect of it. The Lord emphasizes it as a concrete reminder that He calls us into holiness.