Yes, a strange situation to go from being free to becoming a slave. Yet this is what Romans 6:15-23 is all about. In fact, one translation has this heading over the passage: From Slaves of Sin to Slaves of God. Verse 18 seems to be the key verse: "And having been set free from sin, [you] have become slaves of righteousness" (ESV).
The King James uses the word "servants" instead of "slaves". The word means "to enslave", so many recent translations used the word "slaves" instead of the more genteel KJV word of "servants". Robinson's Word Pictures notes that "You have simply changed masters, no longer slaves of sin (set free from that tyrant), but you are slaves of righteousness. There is no middle ground, no 'no man's land' in this war."
"But thank God you've started listening to a new master, one whose commands set you free to live openly in his freedom!" (verses 17-18 Message Bible). Are you listening? Are you living in His freedom? Many Christians love to hear about the freedom purchased for them by Christ on the cross of Calvary. They love to hear of the benefits of the gospel, but not the responsibilities of being a child of God. The altars in the church are not being swamped by people wanting to become slaves of God. "Everybody wants the milk; no-one wants to milk the cow."
Phil Kerr was the leader of a very innovative choir from the mid 1940s to 1960 called Phil Kerr's Harmony Chorus and leader of the Monday Night Musicals in Pasadena, California. He also was a prolific song writer. His song "I'm His to Command" shows his insight to the truth I'm discussing.
"If the dreams that I dream would come true,
and the schemes that I scheme I could do,
then I know there would be
contentment for me.
But I know that He knows what is best for me.
So I'm His to command where He leads me..."
"But now since you have been set free from sin and have become the slaves of God, you have your present reward in holiness and its end is eternal life" (Romans 6:22 Amplified).