We need to consider the warnings to the Hebrews.
The unbelief in Heb. 3 is not unforgivable. Heb. 6 describes those who have been enlightened, etc, falling away to a state where they cannot repent. That reads like a sin that cannot be forgiven
in this life, not a final state of unbelief.
Heb. 6: 4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 and have tasted the good word of God, and the powers of the world to come, 6 if they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame. 7 For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: 8 but that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
Likewise Heb. 10 considers wilful sin after receiving knowledge of the truth, again before death.
26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27 but a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28 He that despised Moses’ law died without mercy under two or three witnesses: 29 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? 30 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. 31 It is a fearful thing to fall into the hands of the living God.
Do these passages indicate that saved sinners can turn away & not just be lost, but become in a state where forgiveness & further repentance is impossible?
Does that apply to the warnings to the Hebrews who saw Jesus & his miracle? Those Jews, Jewish leaders including Pharisees & priests, who rejected their Messiah were commanded by the Apostles to repent, & many did. Acts 2:37-42, Acts 4:10-12, Acts 6:7, Acts 15:5.
Or to the Galatian Gentile coverts? Gal. 1:6-9, Gal. 4:8-11, Gal. 5:1-4.
I believe that once we are saved, born of the Holy Spirit, we
cannot be lost, but the warnings must be taken seriously. In particular, it would appear that we can have a full assurance of salvation that is false assurance, resulting in those who believed they were saved, and accepted by others as saved, are living in a state of deception to such an extent that turning away renders them unforgivable, beyond repentance.