If a person feels they have committed the unpardonable sin, what is their incentive to go back to Christianity as there is no salvation in it for them? Hebrews 6 talks of people that can't be forgiven. Why would an apostate return to Christianity knowing that the Bible dooms them to Hell?
I have 2 points concerning the unpardonable sin/falling away and 2 followups.
1.The definition of the unpardonable sin is plainly defined in the Bible. It does not really matter what we think it is, but how it is defined.
MK 3:28 I tell you the truth, all the sins and blasphemies of men will be forgiven them. 29 But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin."
MK 3:30 He said this because they were saying, "He has an evil spirit."
In verse 30 Christ defined the unpardonable sin as when one thinks that Christ has an evil spirit.
In other words, when one truly thinks that demons are speaking through Christ and/or the miracles Christ made were not through Holy Spirit, but through evil spirits.
Such a person cannot be saved. He is spiritually "insane". And that insanity took a long time to develop by active rejection, knowing slandering and lies about Christ.
2. Concerning Hebrews 6.
HEB 6:4 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 if they fall away, to be brought back to repentance, becauseto their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Here we see the complete itemized list (the underlined) of what one must experience and then fall away in order not to be able to repent.
How do you believe that you have experienced all 6 underlined points?
3. The good comparison of two people that tried repenting after falling away are Peter and Judas Iscariot.
Peter denied Christ 3 times and then went back to his fishing business after Christ's death, "fell away".
Judas was remorseful for betraying Christ and then hung himself.
To a naked eye, Judas' repentance appears to be more powerful than that of Peter.
However, it is not so.
The difference between Peter and Judas is that Peter believed Christ, Judas did not.
Peter accepted Christ's authority, Judas did not.
Peter denied Christ because of simple fear and "fell away" by going back to his fishing business.
Judas denied Christ because he simply did not like his teachings.
Why was Judas so remorseful then?
Because he realized that he betrayed a righteous man who loved him. But he still did not believe in Jesus.
Please tell me, are there things that you flatly disagree with as far as Christ's words go concerning eternal life?
4. There is also a rule of a thumb.
The apostates simply do not care for salvation. A truly unrepentant person simply does not care for salvation. They do not believe in it.

It is a fairy tale for them.
If falling away bothers a person, he is more of a Peter for whom Christ came back.
Thanks,

Ed