We, however, do not belong [to the people who] shrink back for destruction, but [to the people] of faith for the safekeeping of the soul. Hebrews 10:39
Yet despite their privileged status as Christ’s holy brothers, they may not presume on God’s grace, for their salvation depends on their faithful adherence to God’s Son. Since they share in Christ and his inheritance, they dare not renounce and reject him. That is by far the worst act of sacrilege, akin in gravity to trampling him under their feet. Just as they have the promise of eternal life through faith in him, they face eternal death if they disown him. They therefore need to accept God’s judgment of their sins, repent of them, and seek to do God’s holy will; otherwise they may slide down the slippery slope that leads from impenitence to inattention to God’s voice, from inattention to resistance of God’s Holy Spirit, from spiritual resistance to hardness of heart, from hardness of heart to boredom with God’s holy Word, from boredom to gradual withdrawal from the holy community, from withdrawal to the eventual rejection of God’s holy Son. Their remembrance of God’s just judgment keeps them focused on their need for persistence in faith for as long as they live.
John W. Kleinig Concordia Commentary: Hebrews 536