What does Hebrews 6:4-6 mean?

Passages like this illustrate that hermeneutics as it is practiced is not a science, since the results are not reproducible. Determining the meaning of this (nor any) passage cannot be done in isolation of what the rest of scripture says.

If one has concluded from the rest of scripture that salvation cannot be lost, then this passage is made to support it by making the "if" of falling away a hypothetical "if".

If one has concluded that salvation can be lost from the rest of scripture, then this passage is made to support that position by interjecting an intermediate stage of "backsliding" where one is in jeopardy but not fully reprobate, in order to provide that those who have backslidden can regain their salvation.

The stated meaning of the text reflects more upon the interpreter's presuppositions than upon the "science" of hermeneutics.

If one lets scripture speak, then this is a parallel passage to these:

   As a dog returneth to his vomit, so a fool returneth to his folly. —Proverbs 26:11

   But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire. —2 Peter 2:22

As such, the nature of the one "falling away" is revealed by the falling away. He is metaphorically a dog or a sow.

This still does not resolve the issue for those who hold presuppositions. It merely pushes the question to another layer.

"Can a dog or a sow become a sheep?" The answer has to be emphatically "Yes!" Since even a "camel can go through the eye of a needle" with God. In the metaphor of sensus plenior, we are each both sheep and goats. Our goat nature will be separated from our sheep nature when Christ takes us away. He separates the sheep from the goats.

Are we also dogs and sows? The dog is one who returns to his old sin. Paul tells us:

   For that which I do I allow not: for what I would, that do I not; but what I hate, that do I. —Romans 7.15

And the law tells us that we are "unclean until the evening".

The sow is one who does not discern what she is eating as a metaphor for the junk that we bring into our lives. If we still sin in some of the old ways, and we still put junk in our lives, the we too are dogs and sows, even while being sheep.

In the context of the metaphoric meanings, this passage says that the current nature of a person is revealed in their falling away, and that the fallen flesh nature of man cannot be redeemed.

Not mentioned in the passage is that Christ can still make you a new creature.

   Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. —2 Corinthians 5:17

   For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature. —Galatians 6:15

God does not redeem a prostitute. He makes her a virgin.

Adam is an example of one who fell away. He could not be "fixed" which is why we must all be made new. Was he saved? His sin was covered with a promise of Christ in the type of the slain animals for clothing.