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Hebrews 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.
The writer is referring to those in the first century who received Holy Spirit gifts by the laying on of the apostles’ hands. The gifts were supernatural powers, a foretaste of the powers they will have when they are made equal to the angels in the kingdom of God (the world to come). The principle is:
Luke 12:48... For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.
Because God had blessed them above others, he expected more from them. Having believed and been baptised into Christ and been able to exercise God-given powers that men do not ordinarily possess, if they then turned back to a sinful way of life they could not expect a second opportunity of repentance to life.
By their actions they would metaphorically put Christ (who dwelt in them by the spirit) to death in themselves, bring shame upon the name of Christ and cause him to be evil spoken of. It is not a case of the sin which besets us all (for which Christ is our advocate) but rather a wilful return to their old lifestyle, from which a second repentance was not possible.
Romans 6:21 What fruit had ye then in those things whereof ye are now ashamed? for the end of those things is death.
Paul says this
1 Corinthians 9:27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
The writer to the Hebrews returns again to the same principle in chapter 10.
Hebrews 10:26 For if we sin wilfully (NRSV willfully persist in sin) 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?
Holy Spirit gifts could not be given except by the laying on of an apostle’s hands, as is made clear in Acts 8.
Acts 8:18 And when Simon saw that through laying on of the apostles’ hands the Holy Ghost was given, he offered them money,
With the death of the apostles the Holy Spirit gifts ceased. Hebrews 6:4-6 does not apply to us today because we do not possess powers of the world to come. While we have life we can have hope in the mercy of God. Nevertheless putting our hands to the plough and turning back is not to be recommended.
Luke 9:62 And Jesus said unto him, No man, having put his hand to the plough, and looking back, is fit for the kingdom of God.
2 Peter 2:20 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. 21 For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. 22 But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
It is because of this responsibility that Paul says:
Philippians 2:12 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
13 For it is God which worketh in you both to will and to do of his good pleasure.
I hope this helps.