What does it mean to be set free from something (sin in this case), to become slaves to something else (God in this case)? Are we really being set free if we become slaves to something else? The answer is YES! Lets go back to what Paul said in verse 13 "Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness."
Then, he takes this a step further in verse 16 "Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness?"
Ultimately, we serve what we follow - either God, or something other than God - and anything that we follow for purposes other than God's glory is sin. So, we either follow God as His children, driven to serve and please Him as our loving, merciful, gracious Father...willing 'slaves' to the One who saved us - OR - we purse something other than Him, driven by our flesh, our selfish desires, our prideful lusts, willing 'slaves' - loving and serving that which will ultimately destroy us.
Through Paul, God continually brings it up...
In verse 21, Paul talked about the fruit that comes from being a slave to sin, that being death...eternal death and condemnation is the guaranteed result for every person who dies having not entered into a relationship with God, having not been set free from the bondage of sin.
Now, Paul talks about the fruit that flows from a life that has been set free from sin, free from condemnation and bondage, free to experience a relationship with God. "The fruit you get leads to sancitification"...what does that word sanctification mean?
The Greek word here is also translated as 'holiness' and is defined as: purification, i.e. (the state) purity; a purifier:—holiness, sanctification.
To be 'sanctified' by English definition means: to be set apart for a sacred purpose, to be free from sin, to impart or impute sacredness or respect to.