Matthew Henry Commentary Romans 6:16-20

"Know ye not, that to whom ye present yourselves [as] servants unto obedience, his servants ye are whom ye obey; whether of sin unto death, or of obedience unto righteousness? But thanks be to God, that, whereas ye were servants of sin, ye became obedient from the heart to that form of teaching whereunto ye were delivered; and being made free from sin, ye became servants of righteousness. I speak after the manner of men because of the infirmity of your flesh: for as ye presented your members [as] servants to uncleanness and to iniquity unto iniquity, even so now present your members [as] servants to righteousness unto sanctification. For when ye were servants of sin, ye were free in regard of righteousness." — Romans 6:16-20 (ASV)

Every man is the servant of the master to whose commands he yields himself; whether it is the sinful dispositions of his heart, in actions which lead to death, or the new and spiritual obedience implanted by regeneration.

The apostle rejoiced that they now obeyed from the heart the gospel, into which they were delivered as into a mold. As the same metal becomes a new vessel when melted and recast in another mold, so the believer has become a new creature.

And there is a great difference in the liberty of mind and spirit—so opposite to the state of slavery—that the true Christian has in the service of his rightful Lord, whom he is enabled to consider as his Father, and himself as his son and heir, by the adoption of grace. The dominion of sin consists in being willingly slaves to it, not in being harassed by it as a hated power, struggling for victory. Those who are now the servants of God once were the slaves of sin.