Their seems to be a lot of confusion and different interpretations on these scriptures. There are some that believe and even teach that once you are saved if you sin or backslide that you can't be forgiven. That is not what these scriptures are saying.
Let's start by reading Hebrews 6:1-6
Heb 6:1 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
Heb 6:2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
Heb 6:3 And this will we do, if God permit.
Heb 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Heb 6:5 And have tasted the good word of God, and the powers of the world to come,
Heb 6:6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
To understand these scriptures it helps to look at a little bit of the history of the book of Hebrews.
Nelson's Illustrated Bible Dictionary gives us some information on the Historical Setting of Hebrews. It says:
"The repeated use of Old Testament quotations and images in Hebrews suggest that the people who received this book had a Jewish background. The repeated warnings against spiritual unbelief reveal that the readers of this epistle were on the verge of renouncing the Christian faith and returning to their former Jewish ways."
It is important to note when theses letters were written and who Paul was writing to. As near as history can tell Hebrews was wrote sometime before 70A.D. The crucifixion of Jesus occurred 30A.D. We can then see by these dates that the letters were wrote to a people that were alive before and after the crucifixion of Jesus.
Paul was preaching this message to Hebrew Christians that had been raised under Judaism and had been converted from Judaism to Christianity ( Judaism is believing in God but not in Jesus) but he was having problems with them wanting to convert back to there old Judaism ways.
Hebrews 6: versus 1-3
For a better understanding of versus 1-3 it helps to refer back to Heb 5:12
Heb 5:12 says: For when for the time ye ought to be teachers, ye have need that one teach you again which
be the first principles of the oracles of God; and are become such as have need of milk, and not of strong
As we can see from Heb 5:12 Paul was explaining to these people that they should be growing spiritually and shouldn't need to continually be taught the basic principles of the doctrine of Christ. They should have been far enough along in their Christian experience that they should have been teaching others.
Hebrews 6: versus 4-6
It seems as though there has been a lot of confusion about the true meaning of versus 4-6 but they can be easily understood if we look at the time frame in which they were wrote and refer to other scriptures in chapters 9 & 10.
As we mentioned earlier Paul was writing to a people that were alive before and after the crucifixion of Jesus.
Since these people were born under the old law they were raised and taught that an offering be made with the blood of goats and calves for the remission of sins according to God's word.
Now these people were also alive when Jesus was crucified and they were enlightened to the fact that Jesus died for our sins and that he paid the supreme sacrifice with the shedding of his blood for our sins. The problem that Paul was having was that if these people committed sin they were trying to go back to there old Judaism ways and make sacrifices with the blood of bulls and goats. We know that it is impossible for the blood of bulls and goats to take away sins. If we look at some scriptures in chapters 9 & 10 we can see that Paul is once again trying to set them straight.
Heb 9:12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
Heb 9:13 For if the blood of bulls and of goats, and the ashes of a heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:
Heb 9:14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
He was really trying to get through to them as we can tell from these scriptures that the shedding of the blood of Jesus was a far greater, precious and final offering. He continues in Hebrews 10:4 by telling them there is no other sacrifice for sins.
Heb 10:4 For it is not possible that the blood of bulls and of goats should take away sins.
So we can see in Heb 6:4-6 that Paul was telling them that it is impossible to be forgiven for sins by using the blood of bulls and goats. Not only is it impossible but by trying to do so they were putting Jesus to an open shame by indicating that the blood of bulls and goats was more effective than the blood of Jesus.
Lets take a look at Heb 10: 26
Heb 10:26 For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins.
He wasn't telling them that if they sin willfully that they couldn't be forgiven, he was telling them that they couldn't be forgiven by trying to use a different sacrifice.
Christains shouldn't sin but if they do they can be forgiven. Look at 1John 2:1
1Jn 2:1 says: My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous
The Apostle John is telling us that we shouldn't sin but if we do then we can ask Jesus (the mediator between us and God) for forgiveness.
Understanding Hebrews Chapter 6 Versus 1-6