The Greek verb douloo means to enslave, reduce to slavery, or bring into bondage. It appears in Paul's letters describing both the bondage of sin and, paradoxically, the voluntary enslavement to righteousness and to God that characterizes the redeemed life.
Paul's use of douloo in Romans 6 is rhetorically brilliant. He argues that all people are enslaved to something — either to sin or to God. 'You have been set free from sin and have become slaves to righteousness' (Romans 6:18). The freedom Christ brings is not autonomy but a transfer of masters: from a cruel master (sin, which pays death) to a gracious one (God, who gives eternal life). In 1 Corinthians 9:19, Paul says he makes himself a slave (edoulosa) to everyone for the sake of the gospel — voluntary, love-driven servitude that images Christ's own kenosis (Philippians 2:7).