It's a very dirty trick! You can lose your salvation by the choices you make and see no need to repent because of this false sense of security.
I think for us to understand Hebrews 6, we have to go back to the previous chapter. In Chapter 5, he is talking about believers young in the word requiring milk (the first principles) vs seasoned believers on meat.
12For 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 meat.
So in Chapter 6, he goes on to to talk about those already past the first stages:
1Therefore 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,
2Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
So once you're already enlightened and have been so close to God that you've tasted and seen that the Lord is good so much so that you can anticipate heaven:
4For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5And have tasted the good word of God, and the powers of the world to come,
And IF you turn away from all of that, how will you be brought again to repentance? What would now convince you like when you were first enlightened to repent?
6If 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.
It didn't say that you could not be forgiven. But someone like that renouncing God and all the promises after everything would be hard (no impossible) to bring to repentance because of their own guilt.
I think about Judas who knew Jesus and was so close to all the miracles he performed yet gave up everything even the kingdom for a bit of money. In the end his guilt robbed him of everything.
Now we know that with God all things are possible. But what the apostle is talking about is repentance being impossible because of what it signifies to that man.
It's like when Jesus said that it's impossible for a rich man to enter heaven. Mark 10: 17-31. But Jesus went on to say that it's easier for a camel to go through a needle's eye. When asked who could possibly enter heaven, he replied:
27And Jesus looking upon them saith, With men it is impossible, but not with God: for with God all things are possible.
Rich people will be in heaven, right. But in this case Jesus is talking about how much harder it would seem to that rich person when he considers all that he would have to give up. To the rich person it would be impossible but not with God.