The book of Hebrews was of course written to Christians, but also very much with the doubting Jew in mind. With this observation as part of his basis, John Owen had what seems to be a unique perspective on one particular element of the following warning:
He that despised Moses’ law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith He was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?
“The last aggravation of this sin with respect unto the blood of Christ is that it is that ‘wherewith he was sanctified.’ This is not a real internal sanctification, but a separation and dedication unto God, in which sense the word is often used. Some have thought that this refers unto the person guilty of the sin here insisted on; but the design of the Apostle in the context leads plainly to another application of these words. It is Christ Himself who is spoken of, who was sanctified and dedicated to God to be an eternal High Priest by the blood of the covenant which He offered unto God. That precious blood of Christ whereby He was sanctified and dedicated unto God they esteemed an “unholy thing,” that is, such as would have no effect as to consecrate Him unto God and to His office.”
– John Owen, from his commentary on the book of Hebrews
An earlier section of Hebrews directly confirms this:
For such an high priest became us, who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens; Who needeth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people’s: for this He did once, when He offered up Himself. For the law maketh men high priests which have infirmity; but the word of the oath, which was since the law, maketh the Son, who is consecrated for evermore.
This actually renders the warning much more grave than it sounds on the surface, particularly against the religious deception of antichrists. The writer of Hebrews is saying that this apostasy from Christ is of such a nature that it denies not only the full legitimacy of Jesus’ high-priestly office, but also its absolute uniqueness.
For Christ’s own blood consecrated Him unto an office that can neither be substituted, nor shared with, nor acted within, by any other man. And to do so is to “do despite unto the Spirit of Grace.” For the Holy Spirit is the only rightful person by whose power we can be made partakers in the very same Indestructible Life according to which our High Priest made His intercession.
Therefore it is by the Spirit’s groaning within us that we are continually made both witness and subject to the transaction of Christ’s death and resurrection within ourselves, as we also groan with Him.
By this we eat of His bread, and by this we drink of His cup; for in this we discover and become a functioning member of His living body, which is thereby being perfected.
The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?
1 Corinthians 10:16 (ESV)