For the plainest language:
2Peter
[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.
For a fuller explanation by a man known for his deep explanations:
Hebrews 6:
[1] Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
[2] Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
[3] And this will we do, if God permit.
[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.
These scriptures make it plain that they are talking about men that had received the Holy Spirit, yet later turned away.
This is not a failure of the Holy Spirit, but of men that were turned away of their own lusts.
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