HAVING first shown that His works cannot be ascribed to Satan, Jesus proceeds to utter the most terrible of warnings, because they said, He hath an unclean spirit.
"All their sins shall be forgiven unto the sons of men, and their blasphemies wherewith soever they shall blaspheme, but whosoever shall blaspheme against the Holy Spirit hath never forgiveness, but is guilty of an eternal sin."
What is the nature of this terrible offense? It is plain that their slanderous attack lay in the direction of it, since they needed warning; and probable that they had not yet fallen into the abyss, because they could still be warned against it. At least, if the guilt of some had reached that depth, there must have been others involved in their offense who were still within reach of Christ's solemn admonition. It would seem therefore that in saying, "He casteth out devils by Beelzebub...He hath an unclean spirit," they approached the confines and doubtful boundaries between that blasphemy against the Son of man which shall be forgiven, and the blasphemy against the Holy Spirit which hath never forgiveness.
It is evident also that any crime declared by Scripture elsewhere to be incurable, must be identical with this, however different its guise, since Jesus plainly and indisputably announces that all other sins but this shall be forgiven.
Now there are several other passages of the kind. St. John bade his disciples to pray, when any saw a brother sinning a sin not unto death, "and God will give him life for them that sin not unto death. There is a sin unto death: not concerning this do I say that he should make request" (I John 5:16). It is idle to suppose that, in the case of this sin unto death, the Apostle only meant to leave his disciples free to pray or not to pray. If death were not certain, it would be their duty, in common charity, to pray. But the sin is so vaguely and even mysteriously referred to, that we learn little more from that passage than that it was an overt public act, of which other men could so distinctly judge the flagrancy that from it they should withhold their prayers. It has nothing in common with those unhappy wanderings of thought or affection which morbid introspection broods upon, until it pleads guilty to the unpardonable sin, for lapses of which no other could take cognizance. And in Christ's words, the very epithet, blasphemy, involves the same public, open revolt against good.  And let it be remembered that every other sin shall be forgiven.
There are also two solemn passages in the Epistle to the Hebrews (6:4-6; 10:26-31). The first of these declares that it is impossible for men who once experience all the enlightening and sweet influences of God, "and then fell away," to be renewed again unto repentance. But falling upon the road is very different from thus falling away, or how could Peter have been recovered? Their fall is total apostasy, "they crucify to themselves the Son of God afresh, and put Him to an open shame." They are not fruitful land in which tares are mingled; they bear only thorns and thistles, and are utterly rejected. And so in the tenth chapter, they who sin willfully are men who tread under foot the Son of God, and count the blood of the covenant an unholy thing, and do despite (insult) unto the Spirit of grace.
Again we read that in the last time there will arise an enemy of God so unparalleled that his movement will outstrip all others, and be "the falling away," and he himself will be "the man of sin" and "the son of perdition," which latter title he only shares with Iscariot. Now the essence of his portentous guilt is that "he opposeth and exalteth himself against all that is called God or that is worshipped": it is a monstrous egotism, "setting himself forth as God," and such a hatred of restraint as makes him "the lawless one" (II Thess.2:3-10).
So far as these passages are at all definite in their descriptions, they are entirely harmonious. They describe no sin of the flesh, of impulse, frailty or passion, nor yet a spiritual lapse of an unguarded hour, of rash speculation of erring or misled opinion. They speak not of sincere failure to accept Christ's doctrine or to recognize His commission, even though it breathes out threats and slaughters. They do not even apply to the dreadful sin of denying Christ in terror, though one should curse and swear, saying, I know not the man. They speak of a deliberate and conscious rejection of good and choice of evil, of the willful aversion of the soul from sacred influences, the public denial and trampling under foot of Christ, the opposing of all that is called God.
And a comparison of these passages enables us to understand why this sin never can be pardoned. It is because good itself has become the food and fuel of its wickedness, stirring up its opposition, calling out its rage, that the apostate cannot be renewed again unto repentance. The sin is rather indomitable than unpardonable: it has become part of the sinner's personality; it is incurable, an eternal sin.
Here is nothing to alarm any mourner whose contrition proves that it has actually been possible to renew him unto repentance. No penitent has ever yet been rejected for this guilt, for no penitent has ever been thus guilty.
And this being so, here is the strongest possible encouragement for all who desire mercy. Every other sin, every other blasphemy shall be forgiven. Heaven does not reject the vilest whom the world hisses at, the most desperate and bloodstained whose life the world exacts in vengeance for his outrages. None is lost but the hard and impenitent heart which treasures up for itself wrath against the day of wrath.
 "Theology would have been spared much trouble concerning this passage, and anxious timid souls unspeakable anguish, if men had adhered strictly to Christ's own expression. For it is not a sin against the Holy Ghost which is here spoken of, but blasphemy against the Holy Ghost."--Lange "Life of Christ," vol. 2 pg 269.