The Greek verb douleuo (G1398) means to serve as a slave, to be in bondage, or to give oneself entirely to another's service. In the New Testament it describes both servile bondage to sin and the transformed, willing service of those who belong to Christ.
Douleuo appears in one of Jesus' most absolute pronouncements: 'No one can serve (douleuo) two masters' (Matthew 6:24). The word describes total allegiance. Paul's paradox in Romans 6 redefines freedom: believers are 'freed from sin to become slaves (douleuo) of righteousness.' This is the highest freedom — for only by wholly serving God do humans become fully themselves.