Hebrews 10:26

For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more a sacrifice for sins.

This is a return to the warning uttered in Hebrews 6 regarding the final and total apostasy of persons who were once true Christians, concerning whom it was affirmed that it "is impossible" to renew them. Here, the reason for that impossibility is stated in the fact that the rejection of Christ's one sacrifice can only result in the sinner's being left with none at all, "there remaineth no more a sacrifice"! Of course, it would be a mistake to construe every stronghearted and presumptuous sin as "an eternal sin," although the danger that it might become so should never be overlooked. The impossibility of apostasy, euphemistically called the final perseverance of the saints, is not a teaching of the New Testament; and the acceptance of such a doctrine can quite easily lead to a presumptuous arrogance that issues in eternal death.

Clarke's words here are appropriate:

The case is that of a deliberate apostate - one who has utterly rejected Christ and his atonement, and renounced the whole gospel system. It has nothing to do with backsliders in our common use of that term. A man may be overtaken in a fault, or he may deliberately go into sin, and yet neither renounce the gospel, nor deny the Lord that bought him. His case is dreary and dangerous, but it is not hopeless; no case is hopeless except that of the deliberate apostate, who rejects the whole gospel system, after having been saved by grace, or convinced of the truth of the gospel. To him there remaineth no more sacrifice for sin; for there was but the one, Jesus, and this he has utterly rejected.[36]