Read Romans 6:19 Using Other Translations
I speak after the manner of men because of the infirmity of your flesh: for as ye have yielded your members servants to uncleanness and to iniquity unto iniquity; even so now yield your members servants to righteousness unto holiness.
I am speaking in human terms, because of your natural limitations. For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness, so now present your members as slaves to righteousness leading to sanctification.
Because of the weakness of your human nature, I am using the illustration of slavery to help you understand all this. Previously, you let yourselves be slaves to impurity and lawlessness, which led ever deeper into sin. Now you must give yourselves to be slaves to righteous living so that you will become holy.