In this part of the letter, Paul briefly teaches about the aim of Christian life, namely sanctification. Sanctification pleases God and it also the will of God for them. Paul acknowledges that the Thessalonians had done quite well so far in this matter, but there is still more to be done.
Here, we have the idea of maturity in Christian life, which is also seen in Hebrews (Heb 5:11–14). As new in our faith, we are not yet ready for the full demands of Christian life. It is a process leading to maturity. We constantly need to aim for higher standards till we reach the full maturity in Christ (see Ephesians 4:13; Col 1:28).
For the Thessalonians this journey begins with avoiding “fornication” (1 Thess 4:3). Why? Greek culture allowed moral permissiveness (see 1 Cor 5:1). It is possible to guess that faithfulness in marriage to one partner was not also a part of its moral standards. Moreover, the city was located on an important travel route, and so sex industry had to play its part in the city. Paul demands in the name of God that Christians would go against that current of moral permissiveness. For Paul, lack of control in this matter is a sign of pagan culture that does not know God. It could be put in this way:
Thus, sanctification, taking control of one’s body in the matters of sex makes Christians stand out from other cultures and even religions (1 Thess 4:4–5).
Sanctification plays specific role in relationship between husband and wife. Faithfulness in marriage, deepen by the teaching of the Jewish prophets, expresses mutual faithfulness between one God (the husband) and one people (the wife). In the letter to the Ephesians (Eph 5:22–33) this understanding is transferred unto Christ (the husband) and the Church (the wife). Thus, there is no possibility for cheating a brother/sister in this matter. Faithfulness in marriage is so holy in the eyes of God that whoever breaks it - by having a relationship with a wife of another member of the community - incurs God’s vengeance.
In this context then, unlawful sexual relationship is called “uncleanness”, whereas faithfulness in marriage is called holiness (1 Thess 4:7). To disregard this teaching or to take it lightly equals to disregarding God (see 1 Sam 8:7). To Christ and to come to faith should marked a turning point in one’ sentence life. Since that moment there cannot be any compromise to God’s commandments nor lowering Christian standards of morality. If it happens that means that such a person is not guided by the Spirit of God, given on the day of baptism, but by their own sinful desires.
The meaning of brotherly love can be guessed from verse 10. Their mutual love extends beyond their own community towards the brethren “in all Macedonia”. Paul must refer here to the deeds of charity - financial assistance, hospitality (see Rom 12:13; Phil 4:16). It is interesting to notice that the source of these and other charitable action is God Himself.
Verse 11 is worth particular consideration. So far, we could here only dogmatic statements about one true God, Christ’s resurrection and His second coming. So far the Thessalonians were told to live by high moral standards of the Gospel. Now, Paul goes to something very practical, namely daily work and earning the living. Christian faith cannot be confined to theology and morality. It also embraces such ordinary things like daily work. Work is deeply rooted in the Scripture. Since the story of creation human being was called to work (Gen 1:28). The book of Proverbs has much to say about being diligent in work (Prov 10:4; 13:4). In the New Testament Jesus was known as the carpenter (Mark 6:3) and the carpenter’s son (Matt 13:55). Thus, work and the fulfillment of our daily duties and obligations are also a part of agape-love.
Moreover, this aspect of brotherly love stands as a witness to the outside word - the unbelievers. Paul, in order not to create any obstacles to his work of evangelization, worked with his own hands. By being financially independent from the community in Thessalonica, he was not a burden to them - they did not have to support him financially. In the same way, the Thessalonian Christians by being financially independent from outsiders set themselves ‘free’. Let us keep in mind that many among the Christians were coming from the low classes of society and many of them were either slaves or former slaves (see 1 Cor 1:26). By diligently working with their own hands and earning their own living, Christians could rise to prominence in society and show kindness even to those who do not believe yet in Christ. Such change in their own status, such financial independence could be yet another mean of attracting people to the Gospel.