A.T. Robertson Commentary Romans 6:18

"and being made free from sin, ye became servants of righteousness." — Romans 6:18 (ASV)

Ye became servants of righteousness (εδουλωθητε τη δικαιοσυνη). First aorist passive indicative of δουλοω, to enslave. "Ye were made slaves to righteousness." You have simply changed masters, no longer slaves of sin (set free from that tyrant), but ye are slaves of righteousness. There is no middle ground, no "no man's land" in this war.