The changing of what
Heb6:6 says is really a problem. The terrible practice of changing Scripture to suit one's upbringing in an interpretive tradition is actually disgusting and we should all be working against it.
Firstly, we can see from a few English translations that the word "if" is not actually in this verse. I can confirm it's not in the Greek Text but is simply how the NKJ translators are treating a participle that basically means "falling away". Here's how the BDAG Lexicon defines the word:
to fail to follow through on a commitment, fall away, commit apostasy. Most literally it means to fall aside.
The "impossibility" (to renew to repentance those who fall away} is brought forward to 6:4 in all but the NAS translation below:
YLT Hebrews 6:6 and having fallen away, again to renew them to reformation, having crucified again to themselves the Son of God, and exposed to public shame.
NKJ Hebrews 6:6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
- From the NKJ Bible Notes: Or [and have fallen away].
NET Hebrews 6:6 and then have committed apostasy, to renew them again to repentance, since they are crucifying the Son of God for themselves all over again and holding him up to contempt.
- From the NET Bible Notes: Or "have fallen away."
ESV Hebrews 6:6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.
NAS Hebrews 6:6 and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God, and put Him to open shame.
So, there is no "could" being stated in this verse as in the "if they could fall away" as was earlier inserted by
@Everlasting-Grace.
This word meaning "fall away" is not used extensively, but where it is used gives us a good amount of information as to
the severity of the way it's used in Scripture:
NKJ Est. 6:10 Then the king said to Haman, "Hurry, take the robe and the horse, as you have suggested, and do so for Mordecai the Jew who sits within the king's gate! Leave nothing undone of all that you have spoken." (of what you've spoken, let nothing fall away)
NKJ Ezek. 14:13 "Son of man, when a land sins against Me by persistent unfaithfulness, I will stretch out My hand against it; I will cut off its supply of bread, send famine on it, and cut off man and beast from it. (to fall away [in] violation is how the Hebrew phrase re: persistent unfaithfulness is translated in the Greek LXX)
NKJ Ezek. 15:8 'Thus I will make the land desolate, because they have persisted in unfaithfulness,' says the Lord GOD." (essentially the same Greek meaning as 14:13)
NKJ Ezek. 18:24 "But when a righteous man turns away from his righteousness and commits iniquity, and does according to all the abominations that the wicked man does, shall he live? All the righteousness which he has done shall not be remembered; because of the unfaithfulness of which he is guilty and the sin which he has committed, because of them he shall die. (Greek the violation in which he has fallen away)
NKJ Ezek. 20:27 "Therefore, son of man, speak to the house of Israel, and say to them, 'Thus says the Lord GOD: "In this too your fathers have blasphemed Me, by being unfaithful to Me. (Greek: in their violations in which they fell away)
NKJ Ezek. 22:4 "You have become guilty by the blood which you have shed, and have defiled yourself with the idols which you have made. You have caused your days to draw near, and have come to the end of your years; therefore I have made you a reproach to the nations, and a mockery to all countries. (Greek You have fallen away)
I wouldn't try to soften the scenario being stated in Heb6. In fact, I would recognize that it's in the context of taking very seriously growing up from Christian infancy to Christian maturity by moving on from basic teachings (milk) to advanced teachings (meat) that the mature eat (learn and do). There's also a warning in Heb6:3 about this required Christian growth. The writer is rebuking Christians who should have grown up by now Heb5. Then he talks about this requirement to get to "perfection" (maturity/completion) and then the warning for those who don't get this:
3 And this we will do if God permits. (Heb. 6:3 NKJ).
It's not wise to play games with God. His Children take Him seriously. The falling away discussion is explaining foremost 6:3. IOW this lack of commitment in growing up to spiritual maturity may well be seen as falling away and putting Christ to open shame. As I've said before, Christ did die to make infants, but to bring sons to maturity - to completion - to perfection. At some point our Father may evaluate and say, "enough". Note the BDAG definition re: failing to follow through on a commitment.
I'd be very aware of anyone changing this language for any reason including for making it fit an interpretive tradition. God has told us what He expects of us. Language such as this tells us it's not some game we're playing.