Romans 6:22 (NIV) "But now that you have been set free from sin and have become slaves to God, the benefit you reap leads to holiness, and the result is eternal life."
The noun for "slave" occurs elsewhere--St Paul begins the epistle to the Romans by calling himself "Paul, a slave of Christ Jesus..." using the noun doulos, the regular word for "slave." (The King James version has "servant," which is ambiguous.) Similarly, the epistle to the Philippians begins "Paul and Timothy, slaves of Christ Jesus..."
There are other examples in the epistles, including some not by Paul (such as James and Second Peter).
Paul even says that he is speaking to us in human terms to get his point across in verse 19.
Romans 6:19 "I speak to you in human terms because of the weakness of the flesh. For just as you presented your members as slaves of uncleanness, and of lawlessness leading to more lawlessness, so now present your members as slaves of righteousness for holiness."
Sort of changes the meaning when taken context does it not?!?