Hebrews 10:26 speaks of willfully sinning after knowing grace and forgiveness. I suppose this the same as "backsliding". Is there no hope for the back slider who goes back to their sin, regardless of what that sin is? Is there a limit to God's grace? How many times will the prodigal son be welcomed home? We're told to forgive people 70 times 7 but does Jesus have to do that?
Hebrews 10:26 talks about those people who deliberately keep on sinning or make a practice of sin, a sin that is not an isolated sin in a moment of weakness due to imperfection but an intentional one. And their sins are not the result of ignorance. It's like they do evil on purpose. The Bible compares such a person to a leopard that cannot change its spots. (Jeremiah 13:23). Take the case of Adam and Eve, who directly sin to God. Their sinning was willful and deliberate and therefore inexcusable.
But it is reassuring to know that not everyone who has sinned is beyond recovery. God is very patient and takes no delight in punishing those who have sinned on account of ignorance or in a moment of weakness. They can be forgiven by the power of the ransom sacrifice of His Son as long as they display genuine repentance.
The OP asks, in the context of willful sin and Hebrews 10:26:
How many times will the prodigal son be welcomed home?
26 If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left,
The question lies at the heart of what atonement is and how it works, topics discussed and disputed for centuries. The paradox is that omnipotent immutable God is free to forgive at will (Exodus 33:19) without condition, such as sacrifice. Yet Scripture pervasively records that sacrifice is required for atonement by unlimited God, which is impossible.
Why then might unlimited God require sacrifice, specifically of blood, at all? One analysis is that God doesn't need it, rather it is for man's benefit, to remind man of death and the consequence of breaking covenent with God (i.e. sin). In Abraham's time of patriarchal Israel, some more serious covenants were ratified by preparing a sacrifice, cutting it in two pieces and halving it exactly. This symbolized the seriousness of intent to keep the covenant, because the divided carcass represented what would happen to the parties if they broke oath. They placed their lives at risk, they were pledging their life. Then after they walked between the pieces, the carcass was burned, ratifying acceptance. Abraham's crucial covenant with God was sealed this way (Genesis 15).
Sacrifice as part of atonement can be viewed similarly. A re-ratification of covenant with God for man's benefit as a reminder of what is at stake. What God has always asked of man in restoring covenant by atonement is sincerety in repentance:
Isaiah 1: 11-12 NIV
I am sick of your sacrifices. Don’t bring me any more of them. I don’t want your fat rams; I don’t want to see the blood from your offerings. 12-13 Who wants your sacrifices when you have no sorrow for your sins?
And the remedy in verse 16:
Isaiah 1:16 NIV
16 Oh, wash yourselves! Be clean! Let me no longer see you doing all these wicked things; quit your evil ways.
To address the OP question, Hebrews 10:26 does not concern the limit of God's capacity to forgive. Rather, it concerns the limit of man's capacity to repent. This is demonstrated in Hebrews 6
Hebrews 6:4-6 NIV
4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the powers of the coming age 6 and who have fallen away, to be brought back to repentance.
That is to say, it is not impossible for unlimited God to forgive, rather it becomes increasingly difficult and eventually impossible for man to repent. "....lest any of you be hardened through the deceitfulness of sin.." Hebrews 3:13.
How many times will the prodigal son be welcomed home? As many times as he is capable of returning home in repentance.
In summary then, a cautionary message.
"Willful sinning" is explained in v29, as "spurning the Son of God". The argument is not "If you sin, you are spurning Christ", but "If you spurn Christ, you are sinning."
Taking the epistle as a whole, it seems to be addressed to Christian Jews who are getting impatient with Christ given the lapse of time and are in danger of giving him up and returning to their original reliance on the law of Moses.
The broader context of the book of Hebrews is an extended exhortation to Jewish Christians to endure steadfastly in the faith while undergoing persecution which was in large part at the hands of the Jews. This persecution risked driving people to reject Christ and especially to revert to Christless Judaism.
Hebrews addresses this primarily by drawing from Old Testament passages and proving how they speak of Christ, and really how all of the Old Testament was written about Christ. Thus as the fulfillment of those past things, Jesus is better than the Old Covenant.
By the point of chapter 10 the author is beginning his transition from proving his point to applying it, with the main application being to hold on to Christ with a public witness. In this context, it appears that the author is not necessarily speaking of the common sins of daily life, but specifically the sin of apostasy from Christ through hypocrisy. You can see this more clearly by looking at verses 23 and following.
Some would be tempted to reject Christ outright and return to a comfortable condition of Judaism. Others, would be tempted to not necessarily reject him outright, but to withdraw the profession publicly: to neglect to meet and to live as if Christ had not died. This would be to knowingly reject Christ out of fear even while one knows that Christ really is true. Such a person does not have faith in Christ and thus no longer has an expectation of forgiveness.
Thus the correct application is not that every day Christians run the risk of damnation because we may sin intentionally, since nearly all sin involves some degree of consent to it. Rather, it is a willing reversion from Christ, drawing back (v38) and not following Christ.
Of course there is no limit! What a blasphemy to think that God will not forgive a backsliding! And who does not backslide? Saint John Chrysostom says that we, baptized Christians, himself Chyrsostom included, sin as frequently as there are waves in the Mare Nostrum. Thus, absolutely not! God's forgiveness is not limited.
Then what Hebrews 10:26 means? It means that God through His Son's incarnation, crucifixion and resurrection has done everything for our salvation, nothing more can be done, for the abovementioned is perfectly enough. So, if a person having this perfect means for salvation still wrongly considers that it is not enough and having fallen to sin would expect something still higher and more than that which already has been provided, as if it were not enough, then such an ungrateful, wrongheaded and wrongly desperate nominal Christian will forfeit his salvation not because of God not forgiving him, but because of his very ungratefulness, wrongheadedness, and wrong-desperatedness by which he blocks God's otherwise limitless forgiveness for himself.
Short Answer: There will always be Hope for the back slider. All sins are Willful. There is the willful sin from ignorance of Gods laws There is the Willful sin even with knowledge of Gods Laws.
What does that passage then mean?
For we -- willfully sinning after the receiving the full knowledge of the truth -- no more for sins doth there remain a sacrifice," (YLT)
It means that Repentance is the only way out from damnation not another Atonement. For the atonement has be done once
To reject that atonement is to assume that God has another way to make you right with him. But HE DOESNT!
Forgiveness and reward are different
Many people have this question, which is why Paul addressed it with a great word picture:
1 Corinthians 3:12-15 (NASB, emphasis added)
12 Now if any man builds on the foundation with gold, silver, precious stones, wood, hay, straw, 13 each man’s work will become evident; for the day will show it because it is to be revealed with fire, and the fire itself will test the quality of each man’s work. 14 If any man’s work which he has built on it remains, he will receive a reward. 15 If any man’s work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire.
The Church of Thyatira had a similar problem with one lady doing this and leading others to do this...
Revelation 2:23 (NASB, emphasis added)
And I will kill her children with pestilence, and all the churches will know that I am He who searches the minds and hearts; and I will give to each one of you according to your deeds.
It is our deeds that we get reward for. Forgiveness is available to everyone, but the mentoring and teaching of the New Testament presents this question to people who believe Jesus and have his forgiveness:
Sure, you can be forgiven, but is that all you want?
Lifting phrases and words away from the surrounding verses makes it more difficult to understand the whole. Every sin is a willful sin as we consciously choose to commit the act, or fail to do what we should do. The point of Heb 10:26 is not the sinning but the sacrifice required. Read the better translation in Young's.
For we -- willfully sinning after the receiving the full knowledge of the truth -- no more for sins doth there remain a sacrifice," (YLT)
Keeping the time frame of the author (probably not Paul) in the first century AD, about 65-66 AD (1), then picture the background where that temple in Jerusalem was still in operation. The practice of animal sacrifices were still continuing under the old Law which was about to be abolished (Heb. 8:13). The new converts to Christ were still learning and adjusting to the gospel, the new covenant.
The point of Heb. 10:26 was that there was no need any longer to take an animal to be sacrificed for any other sins that they may have committed after having been immersed into Christ. Their (and our) only sacrifice is Yeshua, our Messiah. There is no other - there remains no other - sacrifice for sin.
So, when we slip, make a mistake and do something we should not have done we sorrowfully, sincerely repent and beg God's forgiveness. For if we are faithful to confess our sins to Him, He is faithful to forgive (1 John 1:19).
If we do not confess our sins, if we are not truly repentant but continue in our sins, then we have scorned Christ and made His sacrifice of no effect. That is not backsliding; that is contempt and outright rejection of Him.
See my post "No More Sacrifice For Sin" for a full explanation of Heb. 10:26 ShreddingTheVeil
Consulting the Jewish library aka Tanach is a bit like Trick-or-Treating. You can reach into the bag and find love, hate, joy, misery, rewards, punishment, fidelity infidelity, etc. If you have a negative personality, you will pull out more negative ideas, but if you are of a positive bent, you will likely find wonderful things.
The author of "To the Hebrews" was apparently a sourpuss. Luther had no use for "To the Hebrews" and rejected it. The fact is that its inclusion in the canon was based on a lie. The Roman cabal lied about Hebrews and said it was written by Paul, even though all of Paul's canonical writings begin with his name and credentials, and even though the text itself says plainly that it was not written by an apostle, let alone Paul:
Hebrews 2:1-4 NASB95
1 For this reason we must pay much closer attention to what we have heard, so that we do not drift away [from it.] 2 For if the word spoken through angels proved unalterable, and every transgression and disobedience received a just penalty, 3 how will we escape if we neglect so great a salvation? After it was at the first spoken through the Lord, it was confirmed to us by those who heard, 4 God also testifying with them, both by signs and wonders and by various miracles and by gifts of the Holy Spirit according to His own will.
Paul would gag if he tried to say those words.
Contention about the suitability of Hebrews as a canonical text did not end after its inclusion by the Romans. History tells us that acceptance of Hebrews as canonical is still a matter of dispute.
Personally, I consider Hebrews to have no authority, and I find the first part of the text to be particularly disingenuous (for example):
Psalms 104:4 NASB95
He makes the winds His messengers, Flaming fire His ministers.
Either he or his LXX reverses this:
Hebrews 1:7 NASB95
And of the angels He says, "WHO MAKES HIS ANGELS WINDS, AND HIS MINISTERS A FLAME OF FIRE."
However, his comparison/contrast of the New Covenant vs Ye Olde Covenant is brilliant. I can appreciate Luther and the Roman cabal's difficulty in letting it go, even though it was not apostolic, and had obvious textual issues and translation mistakes.
So now we are saddled with a profoundly compromised text, based on a corrupt LXX, espousing a "yer only as saved as ya are holy t'day" gospel, claiming to be written by Paul, endorsed by the Roman cabal, and trying in vain to apply Habakkuk 2:4 to our own lives.
Here's the Hebrew:
Habakkuk 2:4 NASB95
Behold, as for the proud one, His soul is not right within him; But the righteous will live by his faith.
And the Greek:
4 If he draws back, my life does not find pleasure in it, but the righteous one will live by my faith.
Brannan, R., Penner, K. M., Loken, I., Aubrey, M., & Hoogendyk, I., eds. (2012). The Lexham English Septuagint (Hab 2:4). Lexham Press.
The author of Hebrews (who is decidedly not an apostle) seems to adhere to the idea of Christian perfectionism. IE: Once you repent, you persist in holiness and never fail. This is not unheard of in the ranks of Christianity. Hebrews takes it to the next level. A single slip (unless in ignorance) condemns even the most apparently holy. It is a faith-crushing text.
All of the texts in the Protestant NT canon beginning with Hebrews were moved to the back of the Protestant Bibles, as Luther ordered them, indicating that they were suspect and distant from the theology of Paul.