What does he mean by “it is impossible to renew them to repentance” (6:6) Does he mean once you sin you cannot repent? God won’t forgive you? Who are these people who seemingly cannot be saved? Are they Christians?
Let’s start with the last question first. Whatever this passage means, it applies to Christians. Only they would qualify as those who have “been enlightened,” “tasted of the heavenly gift” and “shared in the Holy Spirit.” The old idea that once one is saved he cannot be lost (and if he is, he was never saved) must be, then, entirely unbiblical. Older translations render the passage “if they fall away,” but the Greek text lists five particular participles, having been enlightened, having tasted of the heavenly gift, having shared in the Holy Spirit, having tasted the goodness of the word of God, and having fallen away.
Why is the writer disturbed about this?
Because his readers are in danger of this happening to them.
If they fall away, can they come back to God?
Yes, but they can’t come as they are. They have to change first, renouncing their fallenness. God’s people cannot expect to live as they please and remain in his fellowship and grace.
Then, there is another possibility: that the fallen will not be able to renounce their ways. It is possible that a person become so hardened in their sin that they not only cannot give it up, they won’t want to.