Hebrews 6:4-6

For [it is] impossible. There are sins that have no forgiveness (Mt 12:31,32). There are Apostates who can never find a place for repentance, not because of the failure of God's mercy, but because they have destroyed their moral capacity for a heartfelt repentance. Usually the most hardened sinner are apostates.

Those who were once enlightened. Had the light of the Gospel. See Joh 8:12.

Have tasted. Experienced.

Of the heavenly gift. The new life in Christ. Christ "giveth life unto the world" (Joh 20:31).

And were made partakers of the Holy Spirit. The Spirit of God is sent into the heart of all sons (Ga 4:6 Ro 8:9).
And have tasted the good word of God. Fed on that word which is food for the soul.

The powers of the world to come. The miraculous gifts of the Spirit. The "world to come" is used in the sense of the Christian dispensation.

See PNT Heb 2:5.
If they shall fall away. Apostasize from the faith.

To renew them unto repentance. He is so far fallen that he has no capacity left for repentance. Judas the Apostate sorrowed, but his sorrows became despair. There was remorse but not repentance.

Seeing they crucify to themselves the Son of God afresh. By rejecting Christ they place themselves with those who rejected him and crucified him because he affirmed that he was the Son of God. Those meant are not those "overtaken in a fault" (Ga 6:1), or backsliders only, but men once Christian professors who not only turn away from but oppose Christ.
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