“Slaves” is a complex subject with many parts: slaves, servants, serve, bonds, bondsmen, and the ability to sell yourself to pay off debts in Israel. Another part is the Gentile/Roman practices and views that Paul used in writing his Letters. Another aspect to work with is KJV vs NIV and other translations. The word “doulos” is the major word used for slave in the New Testament (see notes in Slave); KJV translate it “servant” while NIV uses “slave”. Then there are about thirteen other words that reflect slavery and serving. I thought I would add chains into the word search but Paul connects then to prisoners more than slaves.
In Romans 6:19 Paul explains why he compares our spiritual life to one of slavery, it was very a common practice. Just because it was a common practice does not mean that it was a good or moral practice. I think that is why slave traders are singled out as someone who will receive harsh judgement.
Doulos is a word that stresses the slave as serving the will of the master. In Israel you could sell yourself to someone and the Jubilee determined the price. (I wish I could say that these men and women were treated better but you need to read Exodus 21.) So, when Paul is using the servant/slave to describe our relationship to sin it is not a pretty one. Sin controls everything in your life because you allow it to have that authority over you.
The verses below are my attempts at paraphrasing.
Romans 6:6– Our body was ruled by sin but we crucified our old self so we are no longer slaves to sin.
Romans 6:16– If you offer yourself to obey someone you are their slave; if it is sin, that leads to death. (17) We use to be slaves to sin but now we obey from our hearts and follow the teaching that has claimed our hearts. (19) Comparing us to slaves is something we can see every day in our lives. We use to offer ourselves as slaves to impurity and we wanted more wickedness. (20) If you do not want Jesus and righteousness just be a slave to sin.
Romans 7:14– I have sold myself to sin as a slave because I am unspiritual, I do not understand the Law of God. (25) My sin nature makes me a slave to the law of sin.
*Romans 8:18 – 27 is part of Paul’s use of the idea of slavery to/because of sin. Creation was also forced into slavery and is not free. It also is waiting for the return of Jesus so it can share in the freedom that is to come.