Paul did not labor long in Thessalonica before he ran into serious opposition. As he went on his second missionary journey (starting in Acts 16), he eventually left Philippi and went more or less along the coast of the Aegean sea until he came to the Macedonian capital city of Thessalonica (today Thessaloniki). Luke tells us that Paul found a synagogue there and “on three Sabbath days he reasoned with them from the Scriptures” (Acts 17:2), proclaiming that Jesus is the Christ. Within this three week period, the Lord brought many to faith, including some Israelites, many Greeks, one of whom was Jason based on his Greek name, and even prominent women of the city. Out of jealousy, however, the Jews attacked Paul and Silas and coerced the government to drive them out of the city (Acts 17:4-9). Even after they went southward to Berea, meeting a group of even more receptive hearers, the Thessalonican Jews agitated the mobs in that city as well, forcing Paul and Silas to flee yet again to Athens (Acts 17:10-15). Paul, therefore, had spent about a month total in Thessalonica, probably late in the year 49 A.D.
A year or two later, the Holy Spirit inspired Paul to write 1 Thessalonians, one of the earliest of all his letters in the New Testament. Though his labor among them was short and stormy, Paul did not write them off as a lost cause. “We give thanks to God always for all of you, constantly mentioning you in our prayers” (1 Thessalonians 1:2). Paul had been “gentle” among them, like a mother with her children, speaking the Word of God boldly despite the harshness of those who opposed him (1 Thessalonians 2:1-8). In sending Timothy to them from Athens, Paul also demonstrated his love for them, not desiring to leave them stranded, but also knowing that he could not personally return (1 Thessalonians 2:17-3:5). Satan–whom he names as working behind the Jews who opposed him!–worked against him, but Paul did not lament or bemoan his fate (1 Thessalonians 2:18). Rather, the Thessalonians were his joy and crown of boasting before Christ Himself.
Thus, he exhorts this congregation he labored in for about a month and had not seen face to face for a year or two, to walk in the way of holiness. Paul taught them, however briefly, the patterns of righteousness, and, encouraged by Timothy’s report of the congregation, urges them to follow them even more. “For this is the will of God, your sanctification” (1 Thessalonians 4:3), a verse especially worthy of engraving upon the memory. Paul more or less defines what he means by sanctification by giving several rather specific examples: abstaining from sexual immorality, exercising bodily (not just spiritual!) self-control, and to not sin against a fellow Christian (1 Thessalonians 4:3-6).
Sanctification, therefore, is not an ill-defined state of being. One cannot simply say, “I am sanctified,” and assume that all that needs to be said has been said. Sanctification, simply put, is conformity to the Law of God, because one who is holy strives to do what is holy. Sanctification, therefore, has degrees. Paul says as much when he calls for the Thessalonians to do what they have been doing “more and more.” Sexual immorality is a clear sign of a decreasing or even a dead holiness. Abstaining from such immorality is conforming ever more to the standard the Lord has given us. Of course, such growth never occurs alone. It is God “who gives His Holy Spirit to you” (1 Thessalonians 4:8). Yet sanctification, unlike justification, is not an “all-or-nothing”; a Christian is called to become holier by imitating Christ.
Sanctification is also concerned with particulars. Paul only gives three examples, but they are relatively specific in terms of content. Paul exhorts them to imitate Christ, but also defines what it means to imitate Him. The call to sanctification should not be vague and nondescript, as if the bare command to “be holy as I am holy” covers the whole. We should not be afraid to descend into particulars, because sexual immorality can be determined on the basis of the Law. In other words, the Law draws lines and defines the boundaries of what is and is not pleasing to the Lord.
Paul also reminds the Thessalonians that “the Lord is an avenger in all these things” (1 Thessalonians 4:6). Failure to walk in the way of God is not a misdemeanor. The one who walks the way of sin will reap the rewards of sin, both in this life and in the judgment to come. Walking in the way of God does not happen purely through human will, of course. Nor does walking in the way of God preclude sin, as if sinlessness were possible prior to death. Yet it is the difference between the one who strives to do what is pleasing to the Lord and the one who does not believe that it is necessary to do so. Forgiveness does not mean lawlessness, and keeping the Law does not mean sinlessness.