Why did Paul make sexual purity the focal point of sanctification in this passage? It has a lot to do with knowing God’s will and obeying or rejecting His will and therefore rejecting God.
For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God; that no one should take advantage of and defraud his brother in this matter, because the Lord is the avenger of all such, as we also forewarned you and testified. For God did not call us to uncleanness, but in holiness. Therefore he who rejects this does not reject man, but God, who has also given us His Holy Spirit. (NKJV)
There are not many verses in the Bible that expressly say something is the will of God. Most often, we have to look at a passage and apply a narrative or some direct teaching to understand what God’s will is. Many times, the verses don’t say that one thing or another is God’s will, but uses other words. For instance, 1 Timothy 2:3-4 says, “This is good, and pleases God our Savior, who wants all men to be saved and to come to a knowledge of the truth” (NIV). Other versions use the word desire. God’s desire and His will are different things. His will is something that is certain, such as everyone who comes to him will not perish (Matt 18:14). However, His desire is not certain because it is obvious that not everyone is saved but that is His desire (1 Tim 2:4). When it is His desire, He gives us freedom to align ourselves with His desire or not. When we don’t conform to His desire, it is still a sinful response because we have chosen against His desire. When it is his will, we have no choice, as He will make certain it happens. We can still sin by struggling against His will but at some point, His purpose is accomplished.
Therefore it must be quite important when we find a passage like this that say it is God’s will and then goes on to elaborate on how that should be done. In this case, God’s will is our sanctification.
But now that you have been set free from sin and have become slaves of God, the return you get is sanctification and its end, eternal life. (Rom 6:22 RSV)
The word for sanctification is hagiasmos. You’ve probably heard this described as a separation from the evil things of the world and dedication or consecration to God. That is the basic definition but there is more. It is also a description of the life that we should be leading once we are separated. While we are set apart at salvation, the process of sanctification is “built up, little by little, as the result of obedience to the Word of God, and of following the example of Christ, Matt 11:29; John 13:15; Eph 4:20; Phil 2:5, in the power of the Holy Spirit, Rom 8:13; Eph 3:16.”
In Romans 6:22, we can see the initial calling when we are set free from sin. We can also see that sanctification is a process because there is a beginning and an end. The end is eternal life. That sounds odd, doesn’t it, an end that is eternal. But that is the way it will be. When we eventually reach heaven, our sanctification stops. When we reach the end of the process we will be completely sanctified, we will be like Jesus. This is the promise we have in Romans 8:29 and 1 John 3:2-3:
Beloved, now we are children of God, and it has not appeared as yet what we shall be. We know that, when He appears, we shall be like Him, because we shall see Him just as He is. And everyone who has this hope fixed on Him purifies himself, just as He is pure. (NASB)
We know that now we are God’s children and that is the position we gained at salvation. What full sanctification will be like is not completely apparent to us but we only know that we will be like Jesus and we know He is completely holy or sanctified. Then our response is to work toward purity because we know that is God’s will.
The Thessalonians were still quite young in their faith and didn’t have the advantage of a long time of teaching from Paul. This may be the reason that he emphasizes the beginning of this step by step process with sexual purity.
Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. (Col 3:5 ESV)
It just wasn’t the Thessalonians who were singled out and warned about sexual impurity, Paul also wrote essentially the same thing to the Colossians (Col 3:5), Corinthians (1 Cor 6:13), Galatians (Gal 5:19), and Ephesians (Eph 5:3). The author of Hebrews also warned his audience about sexual immorality (Heb 12:16) as did Peter (1 Peter 2:11). In fact, the warnings about avoiding sexual impurity go back to the Ten Commandments where it is stated simply, “Thou shalt not commit adultery” (Ex 20:14 KJV). People always look for loopholes so the Law expanded upon that to define sexual immorality in more detail. The legalists would think that as long as they are not having sex with someone else’s spouse it would be OK. So Leviticus 18 prohibits many sexual activities, most of which are incestuous but also includes homosexuality and bestiality (Lev 18:22-23). The reason Paul had to warn people about sexual immorality is also provided in this chapter:
Do not defile yourselves in any of these ways, because this is how the nations that I am going to drive out before you became defiled. Even the land was defiled; so I punished it for its sin, and the land vomited out its inhabitants. (Lev 18:24-25 NIV)
These nations are the Gentiles that Paul refers to in 1 Thessalonians 4:5 as well as all non-Jewish people. Sexual immorality is the common behavior of people who do not follow the Lord, they do these things; they think about them all the time; they talk about them and when they can get away with it, they practice them. You may think that is not true, but my experience in the Navy and later in the blue-collar working world before becoming a Christian tells me that the world’s attitude toward sexuality is more perverse that most Christians would ever suspect. The Hollywood portrayal of this is more accurate than we like to admit. The horrible truth is there but we are warned to stay sexually pure.
It is honorable when we do. It honors the Lord, it honors our spouses, and it honors our children. When we are sexually immoral with another person, it wrongs or defrauds him or her because it shows no respect for that person. It shows no respect for the rest of the family. Paul emphasized this when he said that it wrongs his brother. We are simply using others to satisfy our own lusts and desires. They are no more that objects and we care nothing for their spiritual development and walk with Christ or for the damage that does to the rest of the family when it is discovered.
When we become a nation that acts in this way, the Lord promises to vomit us out of the land (Lev 18:25). Don’t be surprised when it happens to the U.S.
Don’t Reject This
Paul began the chapter by emphasizing that his teaching carried the same weight as if Jesus had instructed them Himself. 1 Thessalonians 4:8 puts a bookend on the passage when he says that rejecting this teaching on sexual immorality is the same as rejecting God. Look at what Jesus said:
He who rejects Me and does not receive My sayings, has one who judges him; the word I spoke is what will judge him at the last day (John 12:48 NASU).
Anyone who is rejecting the clear teaching of the Bible will be judged for that. Does that mean that a person will lose his salvation for rejecting this teaching on sexual immortality? No, the Bible doesn’t teach that. But just as Jesus tied receiving His Word to receiving Him or judgment, then a person who is sexually immoral and continues in that lifestyle is in fact rejecting Jesus. The concept is clearly spelled out by John, “Little children, let no one deceive you. He who practices righteousness is righteous, just as He is righteous. He who sins is of the devil, for the devil has sinned from the beginning” (1 John 3:7-8 NKJV). In vs. 7, the emphasis is on practicing and practicing also applies to sin in vs. 8. So it is that the ones who reject the teaching about immorality are rejecting God.
We can’t ignore the clear teaching of the whole Bible on sexual immorality. If we do, we are rejecting God. Thanks be to God, we can always repent and be forgiven. If we don’t, it just proves we aren’t saved in the first place.
 W.E. Vine, Vine’s Expository Dictionary of Biblical Words, (Nashville: Thomas Nelson, 1985), s.v. “NT38”, Biblesoft.