Excerpt taken from http://cicministry.org/commentary/issue65b.htm
Recognizing allusions may also prove helpful in casting light on passages that are often considered “difficult.” This turns out to be the case with the warning found in Hebrews chapter 10:
For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain terrifying expectation of judgment, and the fury of a fire which will consume the adversaries. Anyone who has set aside the Law of Moses dies without mercy on the testimony of two or three witnesses. How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace? (Hebrews 10:26-29)
While there are several issues evident within this complex passage, I wish to focus on the term “sinning willfully” in verse 26. Many have struggled with what exactly the author of Hebrews is trying to convey with this term. The confusion proceeds from the fact that every “sin” is indeed done “willfully” in the sense that all who “sin” do the act of their own volition, thus “willfully” in one sense of the word. Furthermore, the Apostle John makes it quite clear: “If we say that we have no sin, we are deceiving ourselves and the truth is not in us” (1 John 1:8). Thus, if we are to maintain that the Scriptures are consistent in its unified message, “sinning willfully” must have some other meaning than that noted above. The question then is: “What does the author of Hebrews have in mind when he writes the term: “sinning willfully”?
Light is cast on this difficult passage when we realize that the author of Hebrews is most probably alluding to a distinct yet similar warning found in the Old Testament. In Numbers 15 we find that those “under the law of Moses” were given instruction and warning regarding “sinning unintentionally” and “sinning willfully.” Note the following:
Also if one person sins unintentionally, then he shall offer a one year old female goat for a sin offering. The priest shall make atonement before the Lord for the person who goes astray when he sins unintentionally, making atonement for him that he may be forgiven. You shall have one law for him who does anything unintentionally, for him who is native among the sons of Israel and for the alien who sojourns among them. But the person who does anything defiantly, whether he is native or an alien, that one is blaspheming the Lord; and that person shall be cut off from among his people. Because he has despised the word of the Lord and has broken His commandment, that person shall be completely cut off; his guilt will be on him (Numbers 15:27-31).
Therefore, since the author of Hebrews clearly notes that he is making an analogy to the punishment of those who “set aside the law” (vs. 28-29), we get a somewhat clearer picture of what he means by “sinning willfully.” Since there was a distinction in the Old Testament regarding those who had received the revelation of the Lord through Moses of “sinning unintentionally” and “sinning defiantly,” the author of Hebrews makes the following point analogous to the Old Testament instructions and warning: If those who defiantly spurned the Law after receiving the knowledge of its truth were put to death, how much greater will be the punishment of those who defiantly spurn the Gospel of Jesus Christ after receiving the knowledge of its truth.
After carefully examining the passage and the roots that the author of Hebrews is relating it to, it becomes evident the definition of “sinning willfully” carries with it the meaning of one who defiantly blasphemes the Gospel after accepting the concept that it is indeed true. When this is done, only a fearful expectation of the eternal judgment of God remains for that individual.