John Piper addresses that text, and while he is not infallable, I believe his interpretation is faithful to the whole of scripture.
He explains, "In What Sense Were They “Sanctified”?
Now this third description is very controversial. And I do not claim infallibility for my own interpretation. But I commend it to you as consistent with the rest of the book and the rest of Scripture, I believe. Some take it to mean that you can be truly born again and justified by faith, and on your way to heaven through a life of spiritual sanctification - and yet be finally lost and destroyed by forsaking the truth. Because it says here that these apostates had been “sanctified.”
Others say that the possibility raised here of sanctified people committing apostasy will, in fact, never happen, because those who are truly elect and born again will be kept from apostasy by the work of the Holy Spirit. So no sanctified people ever do, in fact, apostacize. And this prospect in Hebrews 10:26-31 never happens. The elect take heed to the warning and persevere in faith and holiness.
The first of these I think to be untenable in view of what this writer says elsewhere and what the rest of the New Testament teaches about the security of the believer in Christ. In Hebrews 3:14 he says, “For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end” - meaning that if we do not hold fast to the end, then we “had not become a partaker of Christ.” Failure to persevere in faith is not a sign of losing salvation but of never having been a partaker of Christ. And in this same chapter (10:14), he says, “By one offering He has perfected for all time those who are being sanctified.” In other words, there is a kind of true, spiritual sanctification that is sure evidence of being eternally perfected in God’s sight - perfected for all time. God’s justifying, perfecting work is not temporary. And the evidence that it is done, is that we are being truly made holy - sanctified.
So I conclude that the sanctification of verse 29 is not the same as the sanctification of verse 14. The one proves eternal perfection (verse 14) and the other proves great guilt after apostasy (verse 29). What is this fruitless sanctification? It seems to be the religious separation and outward purification that often happens when a person becomes part of the visible church. They come under the influence of truth in preaching and teaching. They come under the influence of love among the saints. They come under the influence of the ordinances and even eat and drink the sacred emblems of Christ’s body and blood. They feel the blowing of God’s Spirit of grace and taste his wooing and winning influences. And in all of this, they are visibly set apart from the world - sanctified the way the people of Israel was sanctified among the nations, even though many of them were faithless. And all of this gracious influence was purchased by the blood of Christ, so that verse 29 says, it was indeed “by the blood of the covenant” that these hypocrites were sanctified."
The whole sermon can be found at
desiringgod.org
(Edited)