The writer wants his readers to weigh for themselves how serious it is to turn away from Christ. He has just reminded them that under the law, "He that despised Moses' law died without mercy under two or three witnesses" (Hebrews 10:28). Barnes thinks the writer probably had in mind those who abandoned the religion of Moses, an offence that carried the death penalty (Deuteronomy 13:6-10). If rejecting Moses brought death, what does a person deserve who rejects God's own Son? With the words "suppose ye," Gill says, "the apostle appeals to the Hebrews themselves" and makes them the judges. The question then names three things such a person does: tramples on the Son of God, treats His covenant blood as common, and insults the Spirit of grace.

The writer answers from Scripture: "For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people" (Hebrews 10:30). Then he states the conclusion plainly: "It is a fearful thing to fall into the hands of the living God" (Hebrews 10:31). But he does not leave his readers there. He reminds them how they once "endured a great fight of afflictions" (Hebrews 10:32), urges them, "Cast not away therefore your confidence" (Hebrews 10:35), and ends with hope: "we are not of them who draw back unto perdition; but of them that believe to the saving of the soul" (Hebrews 10:39).

The law did not allow anyone to be put to death on the word of one person: "At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death" (Deuteronomy 17:6). Gill points to this law here. Since death was already the heaviest penalty this world could give, Barnes concludes that the "sorer punishment" must refer to punishment after this life.

Each phrase in the question is strong. Barnes says "trodden under foot" pictures either conquerors who stood on the necks of defeated enemies, or the way people trample what they despise. "An unholy thing" translates the Greek koinos, common or ordinary: treating Christ's blood as if it were nothing special. Barnes links "the blood of the covenant" to Hebrews 9:16-20, where Moses sprinkled the people and said, "This is the blood of the testament" (Hebrews 9:20). "Done despite unto" is enubrizo, which Thayer's lexicon defines as "to treat with contumely," that is, with insulting scorn. This is the only place the word appears in the New Testament.

This question shows how highly God values what He has given. The warning is heavy because the gifts are great: the Son of God, the blood that sanctified, and the Spirit of grace. The same "how much more" reasoning runs through Hebrews: "How shall we escape, if we neglect so great salvation?" (Hebrews 2:3). Matthew Henry's Concise Commentary describes the sin in view as "a total and final falling away," when people with a fixed will "despise and reject Christ, the only Saviour." Gill calls the Spirit of grace the giver of all grace, who "ought not to be in the least slighted, but highly esteemed and honoured."

The warning sits inside encouragement. Just before it, the writer reminds his readers that "he is faithful that promised" (Hebrews 10:23). Just after it, he says, "we are not of them who draw back" (Hebrews 10:39). The question is meant to keep them holding on, not to drive them away.