“But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, and, having been set free from sin, have become slaves of righteousness.” -Romans 6:17–18
Paul now gives thanks to God for the conversion that has taken place in the lives of these believers at Rome. Once slaves of sin, they have been set free by their heart-deep obedience to the gospel—faith in Christ alone—and are now slaves of righteousness.
By his giving thanks to God, we are reminded that Paul is writing a letter to real people, the Church that is at Rome. As we have been following his argument, Paul has shown that “no one can be a servant to righteousness except he is first liberated by the power and kindness of God from the tyranny of sin.” And now he makes the application personal. Paul acts as a father who praises the good that his child does in order to both affirm him and to teach him specifically what he is being praised for so as to encourage the behavior.
Liberated from sin and death, the Roman Christians are free to live as they were intended to live, not as their fleshy passions led them to live. Once cleaned from the filth and stench of the miry pit, who in their right mind would return again to eat with the swine? Living any other way than as slaves to righteousness would not make sense. Real freedom includes freedom from the necessity of returning to their sin (Cf. Proverbs 26:11).
As Jesus himself taught “If the Son shall free you, you shall be free indeed” (John 8:36).