“26 Dear friends, if we deliberately continue sinning after we have received knowledge of the truth, there is no longer any sacrifice that will cover these sins.
I know many people who have heard “received” the knowledge of God and refused to accept it or believe it just as the scripture implies. It has nothing to do with your works after salvation.
The context doesn’t leave much doubt about what it is about.
“For if we sin willfully after that we have received the knowledge of the truth - If after we are converted and become true Christians we should apostatize, it would be impossible to be recovered again, for there would be no other sacrifice for sin; no way by which we could be saved. This passage, however, like Hebrews 6:4-6, has given rise to much difference of opinion. But that the above is the correct interpretation, seems evident to me from the following considerations:
(1) It is the natural and obvious interpretation, such as would occur probably to ninety-nine readers in a hundred, if there were no theory to support, and no fear that it would conflict with some other doctrine.
(2) it accords with the scope of the Epistle, which is, to keep those whom the apostle addressed from returning again to the Jewish religion, under the trials to which they were subjected.
(3) it is in accordance with the fair meaning of the language - the words “after that we have received the knowledge of the truth,” referring more naturally to true conversion than to any other state of mind.
(4) the sentiment would not be correct if it referred to any but real Christians. It would not be true that one who had been somewhat enlightened, and who then sinned “wilfully,” must look on fearfully to the judgment without a possibility of being saved. There are multitudes of cases where such persons are saved. They “wilfully” resist the Holy Spirit; they strive against him; they for a long time refuse to yield, but they are brought again to reflection, and are led to give their hearts to God.
(5) it is true, and always will be true, that if a sincere Christian should apostatize he could never be converted again; see the notes on Hebrews 6:4-6. The reasons are obvious. He would have tried the only plan of salvation, and it would have failed. He would have embraced the Saviour, and there would not have been efficacy enough in his blood to keep him, and there would be no more powerful Saviour and no more efficacious blood of atonement. He would have renounced the Holy Spirit, and would have shown that his influences were not effectual to keep him, and there would be no other agent of greater power to renew and save him after he had apostatized. For these reasons it seems clear to me that this passage refers to true Christians, and that the doctrine here taught is, that if such an one should apostatize, he must look forward only to the terrors of the judgment, and to final condemnation.
Whether this in fact ever occurs, is quite another question. In regard to that inquiry, see the notes on Hebrews 6:4-6. If this view be correct, we may add, that the passage should not be regarded as applying to what is commonly known as the “sin against the Holy Spirit,” or “the unpardonable sin.” The word rendered “wilfully” - hekousios- occurs nowhere else in the New Testament, except in 1 Peter 5:2, where it is rendered “willingly” - “taking the oversight thereof (of the church) not by constraint, but willingly.” It properly means, “willingly, voluntarily, of our own accord,” and applies to cases where no constraint is used. It is not to be construed here strictly, or metaphysically, for all sin is voluntary, or is committed willingly, but must refer to a deliberate act, where a man means to abandon his religion, and to turn away from God. If it were to be taken with metaphysical exactness, it would demonstrate that every Christian who ever does anything wrong, no matter how small, would be lost.”
http://www.studylight.org/com/bnb/view.cgi?bk=57&ch=10
“4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
9 Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation. 10 For God is not unjust so as to overlook your work and the love that you have shown for his name in serving the saints, as you still do. 11 And we desire each one of you to show the same earnestness to have the full assurance of hope until the end, 12 so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises.” — Heb 6