Paul sustains a fairly thoroughgoing analogy between slavery and the Law, Gospel, Christ, and the Spirit in his letter to the Galatians. He uses the analogy in several ways, beginning—I would argue—with representing His Gospel as a ‘sale’ of Christ to the Galatians. The analogy with slavery allows Paul to make the case that his Gospel was given with full disclosure and was fully justified because Christ is effective.
When sold in the marketplace, a placard was sometimes hung about the neck of the slave, who stood on a box (cf. Lucian, Philosophies for Sale). Roman law required that ‘any serious sickness from which the slave was suffering’ should be stated on the placard. Aulus Gellius reports that formal Roman magistrates established a law regarding the purchase of slaves:
"See to it that the sale ticket of each slave be so written that it can be known exactly what disease or defect each one has, which one is a runaway or a vagabond, or is still under condemnation for some offence. (Aulus Gellius, Noctes Atticae 4.2.1).
If a slave developed a serious illness or died, the purchaser was protected by a law that revoked the sale (e.g., Varro, De re rustica 2.10.5). The courts ruled in this matter according to whether the slave’s illness affected his or her efficiency (Aulus Gellius, Noctes Atticae 4.2.2; Justinian, Digest 21.1.10pr.).
Christ Publicly Displayed and Effective as Crucified
In Galatians, Paul is effectively on trial for selling the church an allegedly defective Christ. He says,
Galatians 3:1 (NRSV) You foolish Galatians! Who has bewitched you? It was before your eyes that Jesus Christ was publicly exhibited [placarded; Greek, proegraphē] as crucified!
In the previous verses, Paul states that righteousness does not come through the Law but through Christ’s death (2.15-21). Thus, the Galatians ‘bought’ Paul’s Gospel of righteousness through Christ with Paul’s full declaration that Christ was crucified. It is as though Paul had written on Christ’s placard, ‘Crucified’. Far from being a defect, however, it was precisely through Christ’s death that righteousness came. Crucifixion was no disqualifying ‘disease’ but the very means by which Christ’s service of righteousness was provided. The crucified Christ was, to be sure, effective, and therefore there was no basis to revoke the ‘sale’. Thus, as though in a court of law, Paul puts the question of efficiency to the plaintiffs:
Galatians 3:2, 5 The only thing I want to learn from you is this: Did you receive the Spirit by doing the works of the law or by believing what you heard?... Well then, does God supply you with the Spirit and work miracles among you by your doing the works of the law, or by your believing what you heard?
Proof that Christ was effective in bringing righteousness came in the giving and receiving of the Spirit. To understand this fully, one must remember Israel’s story of transgression and restoration. God, looking upon Israel’s utter sinfulness (Isaiah 59.1-15), brought his own righteousness to the situation (Isaiah 59.16-19). The Redeemer, says Isaiah, will come and remove Jacob’s godlessness (as the Septuagint puts it; ‘transgression’ or ‘rebellion’ in the Hebrew), make his covenant with them, put His Spirit on them, and place His words (commandments) in their mouths (Isaiah 59.20-21; cf. Deuteronomy 30.8-10, 14). In other words, God does not merely deal with sin through providing a justification for sinners but by further providing a righteousness of God through the Redeemer from Zion and His empowering Spirit. Christ crucified proved to be effective, argues Paul, precisely because the Galatians had received the Spirit. To receive the Spirit, he will argue later in chapter 5, means to receive a new obedience not in the legal code but through the work of God in believers’ lives.
Paul extends his theme of slavery in chapter 4 to the Galatians—to all people in Christ. He says that Christ redeemed those who were enslaved under the Law (Galatians 4.4-5). If we return to following the Law’s codes—observing special Jewish laws such as observation of holy days and circumcision—we return to slavery under the Law (Galatians 4.10; 5.2). Any attempt to find righteousness in the Law means that we separated from Christ and have fallen from God’s grace (Galatians 5.4). However, ‘For freedom, Christ has set us free’ (Galatians 5.1). Christian freedom is not license to do whatever we wish but is a new 'slavery' to Christ. Previous, human, social distinctions are replaced by a transfer to Christ as new master:
Galatians 3:28-29 There is no longer Jew or Greek, there is no longer slave or free, there is no longer male and female; for all of you are one in Christ Jesus. 29 And if you belong to Christ, then you are Abraham's offspring, heirs according to the promise.
Paul’s thoughts about the Gospel in terms of slavery are further expressed in an allegory in Galatians 4.22-31. He contrasts Sarah, the free woman, and Hagar, the slave woman, comparing them to the Gospel versus the Law. Later, he uses the analogy of slavery in an ironic way:
Galatians 5:13 For you were called to freedom, brothers and sisters; only do not use your freedom as an opportunity for self-indulgence, but through love become slaves to one another.
The problem with the Law is that it is only effective as a clarification of God’s commandments but it has no power to effect righteousness. It simply cannot make sinful people righteous (Galatians 3.21). The Spirit, however, is God’s empowerment to produce the fruit of righteousness (Galatians 5.22-23), ‘and those who belong to Christ Jesus have crucified the flesh with its passions and desires’ (Galatians 5.24). Note the switch: while Christ was ‘publically placarded’ as crucified when sold to the Galatians, the Galatians are now his slaves. The irony of this is that this slavery is freedom: they are set free from driving passions and desires that lead to sin and belong to Christ and are slaves to one another. They are freed from the flesh, with its sinful desires, because of the power of the Spirit now at work within them. Christ has, indeed, proved effective in bringing righteousness through the cross and giving of the Spirit.
One final point. Paul describes his own ministry in terms of slavery. He says,
Galatians 6:17 From now on, let no one make trouble for me; for I carry the marks of Jesus branded on my body.
The branding of slaves was not a common practice in Paul’s world, but it would have been known about. It was a practice in Semitic and Oriental but not Greek regions. It established permanent servitude and identified to whom the slave belonged. Xenophon thought it a good idea for state-owned slaves (Ways and Means 4.21). In Elephantine, a Jewish community on the Nile in Egypt, a slave is described as being marked on the wrist with a yod. The Old Testament records the practice of marking a slave who willingly enters permanent slavery in a master’s household by piercing his ear with an awl on the doorpost of the home (Exodus 21.6).
Paul earlier describes his service to God in even stronger terms than slavery:
Galatians 2:19-20 For through the law I died to the law, so that I might live to God. I have been crucified with Christ; 20 and it is no longer I who live, but it is Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
Christ’s ownership, as it were, of Paul is exemplary for every disciple. To claim him is to be claimed by him, to die with him, and to be crucified with him, that his life might be lived in the disciple. This loss of distinct identity and taking on the identity of the Christ, while described as dying and rising with Christ, is also a description of slavery. Without endorsing the practice of slavery, Paul finds it an appropriate way to explain the Gospel of a crucified Saviour, his own service and devotion to Christ as his apostle, and also, by way of example, the Christian life. Ironically, such service (for it is more that than servitude) is actually the greatest freedom.
The irony of the Gospel is that Jesus Christ took on the form of a slave and was crucified to set free those who serve him. The Gospel of Christ came with the full disclosure that this was, indeed, a crucified Saviour. Yet this disclosure was far the opposite from a defect. It was the way in which Christ’s service proved effective. It was the very means by which God provided righteousness to those enslaved in sin. Through Christ comes a new identity, no longer determined by human institutions and distinctions but by identity in Christ. Paul gladly and proudly presented his markings as a slave belonging to Jesus. He writes to the Galatians not to seek to return to a slavery to the Law, for in it is no power to set them free from the service of sins of the flesh. Instead, in Christ, they will find freedom to produce the righteousness of the Spirit of God at work in them. The cross of Christ was no defeat. It was the very victory over sin that released God’s transforming power to enable sinful people who put their faith in Jesus Christ to live righteously before Him.
 William Linn Westermann The Slave System of Greek and Roman Antiquity (Philadelphia: The American Philosophical Society, 1955), p. 99.
 A. Cornelius Gellius, Noctes Atticae, rev. ed. (Cambridge, MA: Harvard Univ. Press, 1946). Online: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Gellius/4*.html (accessed 31 July, 2017).
 Westermann, The Slave System of Greek and Roman Antiquity, p. 99.
 Westermann, The Slave System of Greek and Roman Antiquity, p. 19. It actually contributed to slaves revolting in Sicily in 135-132 BC (Diodorus Siculus 34-35).
 Westermann, The Slave System of Greek and Roman Antiquity, p. 19.