To many, this sounds a lot like God refusing to forgive us, even if we ask. But that’s not the point. Stick with me.
(Heb 6:4-6 ESV) 4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 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.
First, a little Greek. “Impossible” means, well, impossible. It’s the same word used in 6:18 (“it is impossible for God to lie”); 10:4 (“it is impossible for the blood of bulls and goats to take away sins”); and 11:6 (“without faith it is impossible to please him”). It does not mean “really difficult.”
Second, the description “those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come” refers to all Christians. Just as all the Israelites had seen the power of God to rescue them from slavery, the Egyptian army, and the brutal Sinai desert, all Christians have received the Spirit, been enlightened, etc. We’ve experienced what it means to be freed from slavery to sin and to live as children of God.
Third, “fallen away” means “left God so as to become damned.” The same word is used in Eze 14:13 and 15:8 regarding Israel’s rebellion against God that led the Babylonian Captivity and Exile, and the same word (with the ek– prefix) is used in Gal 5:4 of falling away by trying to be saved by works rather than faith in Jesus. The context is more than plain: “fallen away” means “become damned.”
That’s a pretty stark interpretation, but it’s what the text really says. And we will only find the truth of the matter by being entirely honest about what the words mean.
So if someone who was once saved falls away, what happens? Well, the writer says that “they are crucifying once again the Son of God to their own harm and holding him up to contempt.” They gladly accepted the once-for-all salvation promised by the cross, and now they’ve turned their backs on it, as though the cross means nothing to them, as though the sacrifice of the Son of God has no value. They treat God’s salvation as worthless. And the world sees a supposed follower of Jesus treating Jesus with contempt. It’s not a good place to be. In fact, it sounds very much like Heb 10:29 —
(Heb 10:29 ESV) 29 How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace?
It’s the same thought, leading to the same conclusion. Indeed, it’s one of the major themes that runs throughout Hebrews: the generosity of God’s salvation is incredible, but if you spurn it, the results are terrifying.