Jamieson, Fausset and Brown on Romans 6:18

18. Being then--"And being"; it is the continuation and conclusion of the preceding sentence; not a new one.

made free from Sin, ye became the servants of--"servants to"

Righteousness--The case is one of emancipation from entire servitude to one Master to entire servitude to another, whose property we are (see on Ro 1:1). There is no middle state of personal independence; for which we were never made, and to which we have no claim. When we would not that God should reign over us, we were in righteous judgment "sold under Sin"; now being through grace "made free from Sin," it is only to become "servants to Righteousness," which is our true freedom.