A.T. Robertson Commentary Romans 6:16

"Know ye not, that to whom ye present yourselves [as] servants unto obedience, his servants ye are whom ye obey; whether of sin unto death, or of obedience unto righteousness?" — Romans 6:16 (ASV)

His servants ye are whom ye obey (δουλο εστε ω υπακουετε). Bondservants, slaves of the one whom ye obey, whatever one's profession may be, traitors, spies sometimes they are called. As Paul used the figure to illustrate death to sin and resurrection to new life in Christ and not in sin, so now he uses slavery against the idea of occasional lapses into sin. Loyalty to Christ will not permit occasional crossing over to the other side to Satan's line.