Paul is following on from his earlier response at the beginning of the chapter to an expected challenge to what he had said at the end of chapter 5. That if sin increases, so does grace increase. Instead of the question "Shall we go on sinning so that grace may increase?" we now read (verse 15), "Shall we sin because we are not under the law but under grace?"
In response to this, Paul continues on with the metaphor introduced in the first few verses of chapter 6. The analogy of slavery. In verse 6 we read "we are no longer slaves to sin", then in verse 7 "we have been set free from sin", and then in verse 14 "sin shall no longer be your master". Now we see the word 'slave' appear 9 times in 9 verses (v15-23). This is of course something that the Romans and people living in Rome and the Roman Empire know a lot about. Scholars estimate about 10% (but possibly up to 20%) of the Roman empire's population were enslaved. This would mean, for an estimated Roman empire population of 50 million (in the first century AD) between five and ten million were enslaved. This number would have been unequally distributed across the empire, with a higher concentration of enslaved people in urban areas and in Italy. (Source: British Museum)
The structure of this part of chapter 6 sees two sections each focused around a 'but now' statement. v16-18 - we are either slaves to sin or slaves to obedience, but now, thanks to God, we have been "set free from sin and have become slaves to righteousness"! Amazing news.
The second 'but now' is found in verse 22. "When you were slaves to sin, you were free from the control of righteousness" but now "you have been set free from sin and have become slaves of God". The benefit of this is holiness and the result is eternal life. The 'but now' frames how it was and how it is now. Our new situation, our new position under God. This is showing us, in a similar way that we were shown our new state under justification, our new state in sanctification.
This brings the question of "what does sin matter?" right to the front of Paul's hit list. Something he and all Christians have to battle with. The answer here is black and white. Quite literally we have two choices:
Slaves to sin, which leads to death
Slaves to obedience, which leads to righteousness
We as Christians have no alternative. Righteousness comes when it is time to be judged. And judged we will be against God's standard - His holiness. If we are found righteous, it leads to life, through our conduct that is pleasing to God.
Verse 18 tells us that we have no alternative, we have "become", translated as handed over, transformed. We have been transformed from one status to another. Verse 19, transformed from our 'human limitations' or natural weakness. We use to offer ourselves up to acts of impurity. These are shameful things that we try to hide. Hide from others and even from God. It is the darkness and corruption that our old selves were held by - as slaves. As the impurity increased, so did the wickedness. The good news is that we are now righteous, which leads to holiness.
We have been plucked from the field of sin and Satan, and moved to the new field, where Jesus reigns in righteousness, we are now free, free to move away from the first field, moving away from the temptation and the lure of sin.
In verse 21 we see Paul ask another question of the reader. "What benefit did you reap at that time from the things you are now ashamed of?" That is, what did we gain from our old lustful and corrupt life? What gain did we get from living like that?
Before we were Christian we were a slave to sin. We were dead and produced no fruit, only shame, hurt and suffering. But now, we have a new status, free from sin and now slaves to God. We produce fruit and have holiness and eternal life!
Through the chapters we have covered so far we have seen comparisons. Law and grace, flesh and spirit, Adam and Christ. Here again, we have seen additional comparisons: sin and God, death and eternal life, wage earned versus gift received!
The point being made is that God will one day complete what He has already made us, holy and righteous, therefore we should seek now what we will become.
God expects us to obey Him, and our very obedience is the product of His grace. What has justified us still continues to work to sanctify us!
The follow-on issue, and we see it in some churches, with some people, is the mindset of legalism. While fundamental legalism is the end of the spectrum and isn't often seen. The idea is that it is by our effort we gain salvation. This is the basis of all other religions except Christianity. The soft version of legalism however can creep in. That is, we can look to obey God through our own efforts. This leads to self-help and human-centred books and programs. This comes into direct conflict with living by faith and our trust in God and His grace for our salvation.
Instead, be filled with the Spirit (Ephesians 5:18b)
So as we obey Christ we are to remember that we do so, not by our own power, but by the very power that has saved us.
We are to sin no more - rather we are to obey God, through God's strength and not our own.