I believe Hebrews 6:4-6 means exactly what is says; To the Church and to all believers.
 
WARNING.. against the danger of falling away - It is possible to go too far, to the end of committing  the unpardonable sin. A sin unto death.  (The sin that the blood of Jesus will not save you from) Blasphemy against the Holy Spirit.  The Bible suggests that names once written in The Book of Life could be blotted out.

Revelation 3:5
HE THAT OVERCOMES, THE SAME shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.

It is impossible for THOSE  who were once "enlightened"  (who once believed) If  THEY should fall away (How far away?) It is obvious that THEY went too far and now reject the truth that they once believed. In essence their response to The Holy Spirit is now not one of repentance but unbelief - calling what they once believed the truth -  now to be a lie. They exchanged the truth for the lie.  Is this not now the unpardonable sin?

In "Eternity"  we are kept, I believe, from falling away because our Salvation is now made complete in Christ.  He is now  our righteousness - incapable of sinning.  Until then our free will and flesh can lead us away from Christ as backsliders and if not repentant we become hardened in our sin and in danger of falling away completely.
 
Christ is "able" to keep believers from falling away but believers must be willing to stay and not fall away.
 
There are many verses about falling and repenting all of which a believer is to be warned of,  but what of the believer that doesn't listen and continues to fall away? Has God changed ? Is He now NOT JUST to judge each man according to his works?
 
Hebrews 6: (Regarding)

THESE BELIEVERS were not just Hebrews who had heard the gospel and seen signs and wonders  - They WERE MADE PARTAKERS (and a share) of the" HOLY" SPIRIT, tasted of the heavenly gift (a consumer) and once enlightened" (the dead and the living  are not compatible - nor is Holiness and wickedness) If THEY should fall away - (All the Way back) THEY cannot return AGAIN to repent. (They repented already once - as in Salvation) That is why there remains no longer a sacrifice for THEM. It doesn't apply a second time.
 
Revelation 2:5
Remember therefore FROM WHERE you are fallen, and repent, and do the first works; OR ELSE I will come to you quickly, and I will remove your candlestick out of his place, except (unless) you repent.

2 Peter 3:17
And so, dear friends, since you already know these things, continuously be on your guard not to be carried away by the deception of lawless people. Otherwise, YOU MAY FALL from your secure position.  (Danger)

Mark 13:13
And you will be hated by all for my name’s sake. BUT THE ONE WHO ENDURES TO THE END will be saved.
 
We are as secure in Christ as we want to be.  It is not Him who would leave or forsake us but we must be WARNED so that .... we would fear that falling away from Him could become fatal to us.