“For if we are willfully sinning after receiving the full knowledge of the truth, there remains no more sacrifice concerning sins…”(Hebrews 10:26).
The writer of Hebrews is repeating a similar thought that he did in Hebrews 6:4-6, though using different words. This person has “full knowledge of the truth,” and so in a similar manner to Hebrews 6:4-6, this person has been “enlightened” and has tasted the Word of God. There are passages in the Bible which speak of people whose lives are characterized by various kinds of sin, and then goes on to say that those who live like this will not inherit the kingdom of God. These kinds of passages are related to verses like Hebrews 10:26, but they do not specifically refer to people who have once professed to believe the true gospel.
Hebrews 10:26 puts forward a specific instance of those who not only live lives characterized by lawlessness (sin), but who do this kind of sinning in spite of “full knowledge of the truth.”
Many irreligious and unconcerned atheists for example, are not sinning in spite full knowledge of the truth of the gospel, whereas a former professor of the gospel is. And from Hebrews 6:4-6, it does not appear that this former professor (i.e., apostate) even wants to repent of his/her lying profession of the true gospel and renew it to a true profession — the message of verses 6:4-6 is that this is impossible.
“I wrote to you in the letter not to associate with fornicators; and not altogether with the fornicators of this world, or with the covetous, or with plunderers, or with idolaters, since then you must go out of the world. But now I wrote to you not to associate intimately; if anyone is called a brother and is either a fornicator, or a covetous one, or an idolater, or a reviler, or a drunkard, or a plunderer, with such a one not to eat. For what is it to me also to judge the ones outside? Do you not judge those inside? But God will judge the ones outside. ‘And you shall put out from yourselves the evil one'” (1 Corinthians 5:9-13).
This person who had been initially “called a brother” is now repudiating that profession by living a life that is indicative of lostness. Paul says not even eat with such a person, whereas he says elsewhere that in some cases it is alright to eat with unbelievers:
“And if any of the unbelievers invite you, and you desire to go, eat everything set before you” (1 Corinthians 10:27).
1 Corinthians 10:27 excludes those unbelievers who had once made a profession of the gospel but who later repudiated that profession. Matthew 18:15-17 puts forth the possibility that a brother may either be won back (Matthew 18:15) or excommunicated if he will not “hear the assembly” (Matthew 18:17), but the sin here is a sin against a brother that is first to be taken up privately between the two persons and so it is not the public sin set forth in passages like Hebrews 6:4-6, Hebrews 10:26, 1 John 5:16 and 1 Corinthians 5:9-13.
From 1 Corinthians 5:9-13 there indeed could be an initial call to repent, but it appears that when the person does not heed this call, then they are to have nothing to do with them again, so much so that they are not to eat a meal with such a person.
“For if by a recognition of the Lord and Savior, Jesus Christ, they have escaped the defilements of the world, and again being entangled they have been overcome by these, then their last things have become worse than the first. For it was better for them not to have recognized the way of righteousness than having recognized it to turn from the holy commandment delivered to them. But the word of the true proverb has happened to them: ‘The dog turning to his own vomit;’ also, the washed sow to wallowing in mud'” (2 Peter 2:20-22).
This passage, just like Hebrews 6:4-6, Hebrews 10:26, 1 John 5:16 and 1 Corinthians 5:9-13 does not give any indication that those who apostatize from the true gospel have any interest in coming back. It appears that they do not even desire to feign a return to the faith.
EDIT (NOTE): Hebrews 10:26 speaks of “willfully sinning after receiving the full knowledge of the truth.” The willful incestuous sin in 1 Corinthians 5:1-13 was worthy of excommunication. To excommunicate a person is to judge that person unregenerate, and to judge the person unregenerate implies that this sin was willful and not repented of (Matthew 18:15-17; cf. 1 John 2:4-6).
Those who maintain the name “brother” despite a life characterized by wickedness are clearly unregenerate (1 Corinthians 5:11). They are obviously engaging in “willful sin.” But this is not the “willful sin” specified in Hebrews 10:26. The “willful sin” of 1 Corinthians 5:1 might lead to the “willful sin” of Hebrews 10:26, but not necessarily so, since Paul gives possible hope that the excommunicated person may be “saved in the day of the Lord Jesus” (1 Corinthians 5:5).
The “willful sin” of those professing the true gospel in 1 Corinthians 5:1-13 and the related “willful sins” of those professing to know the true and living God, but in works denying Him (Titus 1:16; 1 Corinthians 5:11) are different sins than the explicit, willful, and deliberate sin of renunciation of the true gospel of Jesus Christ in Hebrews 10:26-29. Similarly the “falling away” of 1 Corinthians 5:1-13 is a much different “falling away” than that spoken of in Hebrews 6:4-6. For one says it is impossible to renew to repentance, the other does not say this.
Here is John Calvin on Hebrews 10:26 (my comments interspersed):
“For if we sin willfully, or voluntarily etc. He shows how severe a vengeance of God awaits all those who fall away from the grace of Christ; for being without that one true salvation, they are now as it were given up to an inevitable destruction. With this testimony Novatus and his sect formerly armed themselves, in order to take away the hope of pardon from all indiscriminately who had fallen after baptism. They who were not able to refute his calumny chose rather to deny the authority of this Epistle than to subscribe to so great an absurdity. But the true meaning of the passage, unaided by any help from any other part, is quite sufficient of itself to expose the effrontery of Novatus.
Calvin says “‘as it were’ given up to an inevitable destruction.” But it does not look like it is a figurative “as it were” but a literal giving up to destruction. The Bible talks of those who have fallen away after a profession of the true gospel, and so that is the primary consideration, not whether or not this falling away came after being baptized as an infant for example. Those who rejected the book of Hebrews because they could not refute Novatus and his sect, remind me of those heretics who cannot refute the heretical spin that is forced upon the book of James and so they reject the book of James.
“Those who sin, mentioned by the Apostle, are not such as offend in any way, but such as forsake the Church, and wholly alienate themselves from Christ. For he speaks not here of this or of that sin, but he condemns by name those who willfully renounced fellowship with the Church. But there is a vast difference between particular fallings and a complete defection of this kind, by which we entirely fall away from the grace of Christ. And as this cannot be the case with any one except he has been already enlightened, he says, If we sin willfully, after that we have received the knowledge of the truth; as though he had said, “If we knowingly and willingly renounce the grace which we had obtained.” It is now evident how widely apart is this doctrine from the error of Novatus.”
They gave the seeming appearance of obtaining grace by means of a hypocritical profession of the gospel. But they had not truly obtained grace, for if they had, they would not have been plucked away from that profession of the gospel (John 10:28).
“And that the Apostle here refers only to apostates, is clear from the whole passage; for what he treats of is this, that those who had been once received into the Church ought not to forsake it, as some were wont to do. He now declares that there remained for such no sacrifice for sin, because they had willfully sinned after having received the knowledge of the truth. But as to sinners who fall in any other way, Christ offers himself daily to them, so that they are to seek no other sacrifice for expiating their sins. He denies, then, that any sacrifice remains for them who renounce the death of Christ, which is not done by any offense except by a total renunciation of the faith.”
The apostle John writes:
“My little children, I write these things to you so that you do not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous. And He Himself is the propitiation relating to our sins, and not relating to ours only, but also relating to all the world” (1 John 2:1-2).
When Calvin says that Christ “offers himself daily to” sinners I don’t know whether he means sinners who have already made a profession of faith or all sinners without exception who have not sinned in the way described in Hebrews 10:26. If it is granted that by “sinners” Calvin means elect sinners, there are other undeniable statements where Calvin will say that 1 John 2:1-2 is speaking of every sinner without exception.
“This severity of God is indeed dreadful, but it is set forth for the purpose of inspiring terror. He cannot, however, be accused of cruelty; for as the death of Christ is the only remedy by which we can be delivered from eternal death, are not they who destroy as far as they can its virtue and benefit worthy of being left to despair? God invites to daily reconciliation those who abide in Christ; they are daily washed by the blood of Christ, their sins are daily expiated by his perpetual sacrifice. As salvation is not to be sought except in him, there is no need to wonder that all those who willfully forsake him are deprived of every hope of pardon: this is the import of the adverb ἔτι, more. But Christ’s sacrifice is efficacious to the godly even to death, though they often sin; nay, it retains ever its efficacy, for this very reason, because they cannot be free from sin as long as they dwell in the flesh. The Apostle then refers to those alone who wickedly forsake Christ, and thus deprive themselves of the benefit of his death.”
John Calvin believed that Jesus Christ died in some sense for the reprobate. Calvin destroyed the virtue and benefit of the atonement by believing in a universal atonement (not of the exact same kind as the Arminians of course, but a universal atonement nonetheless).