Paul reminds his readers of a general principle of life – if a person chooses to serve another power he will be a slave to that authority. In the spiritual life, we are either slaves of sin or slaves of righteousness.
True believers in Jesus will choose to obey from the heart the standard of teaching that was conveyed to them. Each of them is given a new heart that will want to obey this standard. As far as Paul was concerned, a life of obedience could only happen when a person had received a new heart.
For us to say that Jesus is Lord is for us also to say that we are his slaves. The two go together and are found in every true Christian. It is the case that their lives are not sinless and perfect. Nevertheless, they are described by Paul in verse 19: ‘For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness, so now present your members as slaves to righteousness leading to sanctification.’
How do we show that we are Christ’s slaves? We do so by refusing to serve sin and by choosing to live in a righteous manner. This new lifestyle is a contrast to what we once did (our previous lifestyle is described as lawlessness, which means that our new lifestyles must include obedience to the law) and is comprehensive in scope. A Christian cannot be selective regarding which of Christ’s instructions he should obey and which areas of his life should be given to Christ in submission. Jesus must be Lord of all or else he is not Lord at all.
In addition, this new lifestyle is not static in a spiritual sense. Instead it makes progress in the spiritual life. This progress can be described in several ways. It means to become increasingly like Jesus, it means to have the fruit of the Spirit, it means that there is a growing hatred of and distaste for sin. Sanctification in this sense always describes a changing situation for the better.
Some respond to this description by saying that they are not what they should be as sanctified people. Yet as has often been pointed out, while we are not be what we should be, and while we cannot be what one day we will be (totally holy), we are not what we once were before we believed in Jesus. And any progress we have made is due to the grace of God.