Римлянам 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.

The apostle having at large asserted, opened, and proved, the great doctrine of justification by faith, for fear lest any should suck poison out of that sweet flower, and turn that grace of God into wantonness and licentiousness, he, with a like zeal, copiousness of expression, and cogency of argument, presses the absolute necessity of sanctification and a holy life, as the inseparable fruit and companion of justification; for, wherever Jesus Christ is made of God unto any soul righteousness, he is made of God unto that soul sanctification, Co1 1:30. The water and the blood came streaming together out of the pierced side of the dying Jesus. And what God hath thus joined together let not us dare to put asunder.

6:20 free from the obligation to do right (literally free from righteousness): Paul means either that unbelievers feel no obligation to obey God or that they are unable to do so. But the freedom that they boast of actually makes them slaves to sin.