Therefore let us get past the elementary stage in the teachings about the Christ, advancing on to maturity and perfection and spiritual completeness, [doing this] without laying again a foundation of repentance from dead works and of faith toward God,  of teaching about washings (ritual purifications), the laying on of hands, the resurrection of the dead, and eternal judgment. [These are all important matters in which you should have been proficient long ago.] And we will do this [that is, proceed to maturity], if God permits.  For [it is impossible to restore to repentance] those who have once been enlightened [spiritually] and who have ^[c]tasted and consciously experienced the heavenly gift and have shared in the Holy Spirit,  and have tasted and consciously experienced the good word of God and the powers of the age (world) to come,  ^[d]and then have fallen away—it is impossible to bring them back again to repentance, since they again nail the Son of God on the cross [for as far as they are concerned, they are treating the death of Christ as if they were not saved by it], and are holding Him up again to public disgrace.  For soil that drinks the rain which often falls on it and produces crops useful to those for whose benefit it is cultivated, receives a blessing from God;  but if it persistently produces thorns and thistles, it is worthless and close to being cursed, and it ends up being burned. – Hebrews 6:1-8

It is one thing for a stranger to the faith to resist Christ. But it is another thing for a person who has been in the church and has been enlightened and tasted of the heavenly gift and become a partaker of the Holy Spirit and tasted of the good word of God and the powers of the age to come — it’s another thing for that person to say after all those blessings and all those experiences, “I think what the world offers is better than Christ.” That is a re-crucifying of Jesus and a putting him to public shame worse than any outsider could, who never tasted the truth. – John Piper

How much greater punishment do you think he will deserve who has rejected and trampled under foot the Son of God, and has considered unclean and common the blood of the covenant that ^[a]sanctified him, and has insulted the Spirit of grace [who imparts the unmerited favor and blessing of God]? – Hebrews 10:29

This does not mean that those who stray for a season cannot be restored.  Those who have not completely fallen away, who have not held the Son of God up to contempt, can indeed be restored to repentance.  A sinner who has genuine sorrow over sin shows that the Holy Spirit is working in his heart and is not forever lost.  But those who have indeed fallen away completely are impossible to restore, and God will not permit those who have committed such apostasy to come to repentance.  – Alex Carmichael

If They Fall Away