Romans 6:16

His servants ye are whom ye obey (douloi este wi upakouete). 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.