Romans 6:20

For when ye were the servants--"were servants" of Sin, ye were free from--rather, "in respect of" Righteousness--Difficulties have been made about this clause where none exist. The import of it seems clearly to be this:--"Since no servant can serve two masters, much less where their interests come into deadly collision, and each demands the whole man, so, while ye were in the service of Sin ye were in no proper sense the servants of Righteousness, and never did it one act of real service: whatever might be your conviction of the claims of Righteousness, your real services were all and always given to Sin: Thus had ye full proof of the nature and advantages of Sin's service." The searching question with which this is followed up, shows that this is the meaning.

— Jamieson-Fausset-Brown Commentary (public domain)