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?!?
You are correct that context is important.
As Verginius Rufus, obviously much more of a Bible scholar than I, points out, there are many references where Bible writers refer to themselves and others as slaves of God or of Christ.
As I have pointed out in another post, Bible writers had a very different perspective on slavery than we do. Context, in its broader sense, includes the entire document as well as information about the perspective the writers and original readers would have brought to the work.
Islam, at least originally, had a system of slavery that was much more like that of the Bible than like that of early America. There was little or no racial or ethnic component to the institution.
Slaves often became respected, powerful and wealthy. For instance, Ali became Mohammed's son-in-law, and eventually himself caliph. His assassination led to the blood feud in Islam that still divides Sunni from Shia.