The Greek hekousios means voluntary, willing, or free — done of one's own accord rather than under compulsion. In Philemon 14, Paul uses it to emphasize that he wants Onesimus to help voluntarily, not under compulsion: 'so that any favor you do would not seem forced but would be voluntary (hekousios).' In 1 Peter 5:2, elders are to shepherd 'not because you must, but because you are willing (hekousios).'
The theology of hekousios service and sacrifice stands at the heart of NT ethics. Hebrews 10:26 uses the adverb form for a chilling warning: 'If we deliberately (hekousios) keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left.' This is the flip side: what makes voluntary sin so serious is that it mirrors the voluntary sacrifice of Christ — but in the opposite direction. Christ voluntarily gave Himself; deliberate post-conversion sin voluntarily tramples that sacrifice. The power of hekousios cuts both ways: toward redemption and toward rebellion.