The book of Hebrews, according to John Piper, teaches that salvation will no more be available to the one who falls away. Do you agree with this view?
Go through the entire book, these 'falling away' are described as brethren, partakers of the Holy Spirit, tasted the good word of God, received the knowledge of the truth, sanctified by the blood of the covenant, referred to as 'His people'. They entered not into His rest because of unbelief. Even Moses and Aaron were refused entrance into His rest because of their unbelief.
Check out chapter 10:
26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more a sacrifice for sins,
27 but a certain fearful expectation of judgment, and a fierceness of fire which shall devour the adversaries.
28 A man that hath set at nought Moses law dieth without compassion on the word of two or three witnesses:
29 of how much sorer punishment, think ye, shall he be judged 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?
30 For we know him that said, Vengeance belongeth unto me, I will recompense. And again, The Lord shall judge his people.
31 It is a fearful thing to fall into the hands of the living God. Heb 10
These in chapter 10 had 'received the knowledge of the truth', were 'sanctified by the blood of the covenant', called 'His people'. It's not just 'any ol' sin' that's in view here, but a specific sin (returning to apostate Judaism) of which a specific people (professing Jewish Christians), of a specific time (the very generation that judged Christ to be worthy of death) were under pressure to do.
There was to be no repentance granted for those that fell away and returned back to that apostate system that killed Christ, because by returning they crucified the Son of God afresh and put him to an open shame (Heb 6:4-6), but there was only a certain fearful expectation of judgment which would devour the adversaries before that generation passed away (Heb 10:37), for those that had trodden under foot the Son of God and counted the blood of the covenant wherewith he was sanctified an unholy thing and done despite unto the Spirit of grace (Heb 10:27-29) by falling away back to the apostate Judaism they been delivered from.
THAT is a very crucial message contained within the book, FOR ALL BELIEVERS.