Brothers and sisters, having been set free from sin, you have become slaves of righteousness. I am speaking in human terms because of the weakness of your flesh. For just as you once presented your members as slaves to impurity and to greater and greater iniquity, so now present your members as slaves to righteousness for sanctification.
When you were slaves of sin, you were free in regard to righteousness. So what advantage did you then get from the things of which you now are ashamed? The end of those things is death. But now that you have been freed from sin and enslaved to God, the advantage you get is sanctification. The end is eternal life. For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
Saint Paul has an interesting view on life. Basically, he says that we are always enslaved to someone or something. Always. We are never free, we are never truly independent. But he does specify that he is speaking about slavery in human terms because of the weakness of our flesh, meaning that when Paul compares our life to slavery, he does not exactly mean it literally. At the same time, there is no other way to describe it.
Before baptism, before turning our life to Christ, before becoming members of Christ's Body, our members, our bodies, were enslaved to impurity, sexual and spiritual. But since we are baptized and part of Christ's Church, we should present our members, as slaves, to righteousness. Otherwise, our baptism is kind of pointless. Saint Paul says to present our members "as slaves" because slaves had no choice, but to do the will of their masters. We, as Christians, as servants (in Greek and Hebrew, the word for slave and servant is the same) should act as if we have no choice, but to do the will of our heavenly Father.
Sanctity and righteousness are words that we use in church and our religious life as almost synonyms. And they may be synonyms, but what did they mean initially, when they were used by the people of faith in the times of Christ?
Sanctity or sanctification signify belonging to God, being extracted from the worldly temptations. And righteousness of a Christian means a new lifestyle, that corresponds to the Truth of God, revealed in Jesus Christ. Righteousness is a principle feature of holiness, of sanctified life. The apostle Paul always underlines the extreme difference between former and new life. Former life (or as it is called in our Baptismal service - former delusion) was characterized by "impurity and greater and greater iniquity." Sinfulness leads to more sinfulness, that's just how it works. This is the essence of slavery to sin. Sin produces more sin and death.
However, the new life, the life of "slavery" to God and His truth, leads through sanctification to perfect holiness. Being a realist Saint Paul knew well that a person, who turned his/her life to Christ, did not become a perfect Christian right away. The life of such person was a continual struggle, fight, war even, with sins and temptations of this world. Therefore, Paul borrows from military terminology: "wages" and "free gift."
A soldier received wages for his service, and a free gift was given by the favor of the emperor on special holidays. Apostle Paul juxtaposes the wages of sin (for which we have to literally and painfully work) with the free gift of God (which is given to us for willingly following Christ). God does not reward us for doing good deeds. Rather, we do good deeds because we realize that God has given us a free gift of His grace.
Yours in the Lord,