Heb 6:4-6 But what about people who turn away after they have already seen the light and have received the gift from heaven and have shared in the Holy Spirit? What about those who turn away after they have received the good message of God and the powers of the future world? There is no way to bring them back. What they are doing is the same as nailing the Son of God to a cross and insulting him in public! CEV
The Greek starts with a preposition that means "it is impossible that". At minimum one must realize how serious God is and how important it is that we do not fall away. The writer warns the readers to not decide to quit the church, go back to old beliefs without considering the consequences. The writer often contrasts Jesus' new revelation with the older Jewish revelation. This new revelation of God is much greater, much stronger, much closer to God. And it is not a good thing to give it up, as is implied the people this is written to are considering.
But what about what he writes here? Is it true that once one falls away, repents of being a Christian there is no way back? The Greek uses the word "tasted" twice. Once we have tasted God, can we give it up? Certainly there is only one salvation, only one crucifixion. Jesus will not do it again. It is once and done. But wasn't that one time sufficient, even if we fall away and attempt to come back again?
Perhaps he is speaking about us. How hard will it be for us to repent a second time after rejecting it once? It will take a lot more humility than most people have. The first time took humility but we can tell ourselves we did not know any better. But now we did. We did taste how good God was. And we rejected it for a bowl of porridge. Are we going to be able to fully and honestly repent again? It will be much harder for us a second time. Perhaps that is one way to look at it. But are we being honest with the text?
I think one thing is true. If we decide to fall away to try other things, to sin a lot with the idea of coming back later, one may never get back. That is too risky. Certainly we have been tempted to do some sin: have sex with some person casually, steal money from a business or the government, live a lie with the idea that later God will have to forgive us and we'll be good. There are lots of reasons not to do that but one reason is we may get permanently trapped or we may die while still fallen away. Would God's forgiveness still apply while we are in flagrant sin? The writer here leaves some room for doubt. And I think we should struggle with it, if we are serious about being a disciple of Jesus.
The writer goes on to say those of us who have tasted God but then go on to produce bad fruit, what will God say about that? But after warning them, the writer then encourages them that the works they have done in God's name will be remembered by God. He encourages them to be steadfast, not lazy.
He concludes the chapter by reminded them of the revelation to Abraham and then beginning to show that Jesus is a priest of the order of Melchizedek, whom Abraham acknowledged as an agent of God. That he will continue into the seventh chapter.