29. sorer--Greek, "worse," namely, "punishment" (literally, "vengeance") than any mere temporal punishment of the body.
      suppose ye--an appeal to the Hebrews' reason and conscience.
      thought worthy--by God at the judgment.
      trodden under foot the Son of God--by "wilful" apostasy. So he treads under foot God Himself who "glorified His Son as an high priest" (Heb 5:5; 6:6).
      an unholy thing--literally, "common," as opposed to "sanctified." No better than the blood of a common man, thus involving the consequence that Christ, in claiming to be God, was guilty of blasphemy, and so deserved to die!
      wherewith he was sanctified--for Christ died even for him. "Sanctified," in the fullest sense, belongs only to the saved elect. But in some sense it belongs also to those who have gone a far way in Christian experience, and yet fall away at last. The higher such a one's past Christian experiences, the deeper his fall.
      done despite unto--by repelling in fact: as "blasphemy" is despite in words (Mr 3:29). "Of the Jews who became Christians and relapsed to Judaism, we find from the history of Uriel Acosta, that they required a blasphemy against Christ. 'They applied to Him epithets used against Molech the adulterous branch,' &c." [THOLUCK].
      the Spirit of grace--the Spirit that confers grace. "He who does not accept the benefit, insults Him who confers it. He hath made thee a son: wilt thou become a slave? He has come to take up His abode with thee; but thou art introducing evil into thyself" [CHRYSOSTOM]. "It is the curse of evil eternally to propagate evil: so, for him who profanes the Christ without him, and blasphemes the Christ within him, there is subjectively no renewal of a change of mind (Heb 6:6), and objectively no new sacrifice for sins" (Heb 10:26) [THOLUCK].

JFB.