Paul Kretzmann, the son of a Lutheran pastor, wrote his Popular Commentary of the Bible, which "has been a favorite among confessional Lutherans since publication of the first volume in 1921."
In his comments on Hebrews 6, he called attention to the characteristics of the one who is spoken of in the passage: they were once enlightened, tasted of the heavenly gift, were made partakers of the Holy Ghost, tasted the good Word of God and the powers of the world to come. The description, he says, could only apply to those who were actually saved.
If persons to whom this description applies, people that have undoubtedly accepted Jesus as their Savior, placed their trust in His salvation, and anticipated the joys of eternal life by reason of the power given to them through the Word, now fall away in spite of this saving knowledge, by a deliberate denial of that knowledge, then their return to repentance is excluded. The reason for this fact is not to be sought in God, as though His gracious intention and will in their behalf had not been sincere, but in the people themselves. If their apostasy takes place as here described, with a deliberate, malicious denial of the truth, then they crucify to themselves the Son of God and set Him forth to shame and ignominy before men. They purposely and willfully deny all connection with the Lord, who was crucified for them, they brand Him as a criminal, as a false Messiah, who suffered the disgrace of death on the cross. All this they perpetrate against Him whom they formerly acknowledged as the Son of God, whom they knew to be the Savior of the world. They cannot plead ignorance, or that they acted in foolish unbelief. For that reason their behavior brings upon them judgment, eternal condemnation. Therefore the reason why their hearts become hardened, why it becomes impossible for them to return and to be renewed unto repentance, is to be found in the character of their transgression. They steadfastly and persistently persevere in their antichristian, blasphemous conduct, they harden their own hearts against all attempts of the Word to find an entrance, and are thus finally given over into their hardness of heart.
The writer does not say that his readers have reached this stage; he merely states the possibility that it may happen to them as it has to others, thus warning them to beware of spiritual sluggishness, of lack of diligence in the use of the means of grace. ... If, therefore, any persons that have received these blessings harden their hearts and bring forth fruits of blasphemy and malicious denial of grace, they have sealed their own doom. For the behavior here described is the sin against the Holy Ghost, for which there is no forgiveness, neither in this world nor in the world to come.
In other words, the passage is not describing a mere "falling away" -- Kretzmann's position is that "they crucify to themselves the Son of God afresh, and put Him to an open shame" is not a characteristic of all apostasy, but of a special kind of apostasy: the sin against the Holy Ghost.
Regarding the sin against the Holy Ghost (spoken of in Matthew 12, Mark 3, and Luke 12), he writes:
The sin is committed, not against the person, but against the work of the Holy Ghost, which consists in calling sinners to Christ and giving them the assurance of their salvation. Not the mere blasphemous thoughts, but the actual speaking, the open mockery of the work of the Holy Ghost, is condemned in these passages. If the work of the Holy Ghost is believed to be, and is openly declared to be, the work of Satan, then the blasphemy is directed against the Spirit. Such blasphemy is uttered in full consciousness and with the most perfect comprehension of the import of the blasphemy; the blasphemer glories in his blasphemy. ... Man having gotten into this condition of continual blasphemy by his own fault, repudiates all attempts of God to influence Him for good. The soil of his heart has become cursed, and will bear nothing but thorns. The sin against the Holy Ghost is therefore one which cannot be acknowledged; a confession of sin and a desire for forgiveness is excluded by its nature.
The person living in this sin will continue in his stubborn resistance, with blasphemous, outspoken mockery of the work of the Holy Ghost, until the end. The sin is not unpardonable on account of its greatness, but on account of its nature of rejecting all pardon. No one has committed the sin that still seeks repentance.
Pardon is excluded not by God, but by the apostate himself, as Kretzmann said in both of the sections I've quoted. "Restoration unto repentance" is excluded because such a person will not seek repentance.
So an act of apostasy is permanent if the apostate wills it to be so by continually rejecting and blaspheming the Holy Spirit. An apostate can be restored upon repentance, but one who never repents is never restored.
