ANSWER: Before making any comments on these verses we shall quote them, so as to have them before us.
"For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, but a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. He that despised Moses' law died without mercy under two or three witnesses: of how much sorer punishment, suppose ye; shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? For we know Him that hath said, Vengeance belongeth unto Me, I will recompense, saith the Lord. And again, The Lord shall judge His people. It is a fearful thing to fall into the hands of the living God."
It would be well to keep before us that the Apostle, by the Spirit, was writing to the professed believers from among the Jews. Large numbers of them turned to the Lord from the rituals of Judaism (Acts 2:41; 4:4; 6:7; 21: _ 20), but some of them, because there was no reality in their profession, were in constant danger of apostatizing. The Apostle treated them all as real, but supposed that some might turn back, and consequently gave solemn words of warning. He had painstakingly set forth the immense superiority of the realities of Christianity over the old system of types and shadows. A word frequently found in the epistle is "better"; they had been brought into the better things.
In the 9th and 10th chapters the Apostle had shown the infinite worth and unchangeableness of the one sacrifice of Christ, in contrast with the often repeated and never satisfactory sacrifices of the Mosaic economy. This one "better" sacrifice had forever perfected the true believer. It would never be repeated, and never would need to be.
If a professor of faith in Christ would, after coming to that objective knowledge, turn back, there would be no more sacrifice for sins. His case would then be hopeless; he would have forsaken what he had learned (intellectually at least, we do not say in reality) was the only suitable sacrifice before God.
Such a one turning back would prove that he was never saved at all, and in turning back he would be going on willfully in sin. It would be an entirely different case from one who was saved and fell into some sin or grievous fault. The latter would feel the sin and the dishonor to the Lord, and judge himself for the allowance of the flesh's acting. For such cases there is provision: "If any man sin, we have an advocate with the Father." 1 John 2:1. The advocacy of Christ would be needed for such sin, but relationship would not change; it is still with the Father. And the one who thus falls into sin confesses it—"If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."
The case supposed in Heb. 10 is that of apostasy; that is, forsaking Christ and His one perfect offering. If you speak to a true saint who has failed, and you ask him, Would you give up Christ? he would instinctively say, Oh, no, I could never do that; He is all to me. Herein lies the vast difference between a saint's falling, and a mere professor giving up his profession and turning back to his sins.
The being sanctified by blood mentioned in verse 29 is an outward separation which may or may not be real. When the first covenant was instituted with Israel, Moses sprinkled "both the book and all the people" (Heb. 9:19). They were all marked then, and that by blood; and all who came within the pale of Christian profession were outwardly set apart by blood—the blood upon which the new covenant will yet be established with Israel.
For any who apostatize and forsake Christ, there is nothing left but looking for judgment and fiery indignation. This is reserved for the adversaries, and an apostate places himself among such; in fact, he is apt to become one of the worst. Some of the most bitter enemies of Christ and the gospel were once professors, yes, even preachers of the gospel. Judgment much worse than that which befell the despisers of the law given by Moses awaits all such. It is indeed a fearful thing to fall into the hands of the living God.
There is another solemn reflection in this connection: verse 25 is directly connected with this apostasy. Perhaps their turning back first became apparent when they neglected the public testimony to His name, for it brought shame and reproach that they were not willing to suffer. Therefore we get the solemn exhortation:
"Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching."
The Lord has placed His approval on the gathering together of His saints, and it is in the assembly that He ministers by the Spirit. The Lord never intended that His saints should walk as isolated entities. If one were unavoidably hindered from such assembly the Lord would sympathize with him, but to carelessly neglect such opportunities can only accrue loss to oneself.
We might add one word as to Hebrews 6 where a similar truth is given. In that chapter the apostasy is turning back from the circle where the demonstration of the power of the Spirit of God was witnessed.
We get apostasy today in those who turn back from the profession of an orthodox creed, either to willful sin as the washed sow returns to the mire (note, it was never a sheep, but only a sow all the while), or to the embrace of that which denies the Person and work of our Lord Jesus Christ; such as atheism, modernism, Unitarianism, Russellism ( Jehovah's Witnesses), Christian Science, Seventh Day Adventism, and the like.
There is hope that a mere lifeless professor of Christ may at some time turn to God in true repentance, and with living faith accept the Lord Jesus Christ as his personal Savior, but if he abandons that outward profession and denies either His Person or the value of His atoning work it is apostasy. Therefore let all professors take heed to the words of warning. It is indeed solemn to trample under foot the Son of God and to do despite (to insult) to the Spirit of grace—and all the worse because it is the Spirit of GRACE.