Romans 6
16 Do you not know that to whom you present yourselves slaves to obey, you are that one’s slaves whom you obey, whether of sin leading to death, or of obedience leading to righteousness? 17 But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were [e]delivered. 18 And having been set free from sin, you became slaves of righteousness.

From my bible commentary:
Whatever you present yourself to obey, you become its slave. For example, if I obey my appetite constantly, I am a slave to it. So we have a choice in our slavery: sin leading to death or obedience leading to righteousness.

It is interesting that while we are free to choose, we are not free. We will be a slave to whatever choice we make. Paul’s choice of words here is very interesting.

Matthew 6:24 says something very similar:
No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to the one and despise the other. You cannot serve both God and money.

Romans 6
19 I speak in human terms because of the weakness of your 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 [f]for holiness.