Read Heb. 6:1-3 and try hard to tie it to what the writer is saying in 4-8. Christians, he says, can fail to produce the kind of crop that God intended. And it is futile to think that simple, basic teaching about fundamentals of faith will move them in the midst of their crucifying activity. And, yes they will be destined for fire, unless they are somehow moved to recognize what they are doing. They are dogs returning to their vomit.
But his current audience is not to that point of crucifying their Savior again. So, he looks forward, God willing, to extending their knowledge and their faith toward perseverance.
So, in context, it is not for all time impossible for them to come to repentance, just not by simple truths presented in their present frame of mind and action.
This is saying based on your understanding of the text, is if you lose your salvation.....you cannot get it back.
Correct me if I've mis-stated your position on this.