Commentaries:
Robertson's Word Pictures (NT)
Ye became servants of righteousness (edoulwqhte th dikaiosunh). First aorist passive indicative of doulow, 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.
Other Robertson's Word Pictures (NT) entries containing Romans 6:18:
John 8:32
Romans 6:19
Romans 6:19
1 Corinthians 7:22
2 Peter 2:19
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