It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Paul jumps right into building on that foundation by looking at the question of losing your salvation. This passage has been misused and abused to beat down Christians and to stop them from living in the victory that is Jesus Christ. There are many people who use this passage to say that you can lose your salvation if you don't do these things or if you do other things but it is not even talking about salvation. He is talking to people that have accepted the grace of God through Jesus Christ and so are saved as he speaks of tasting "the heavenly gift". He mentions the fact that they "have shared in the Holy Spirit" which reminds us that one of the purposes of the Holy Spirit is to give us the comfort and assurance that we are saved. So, what does he mean by "if they fall away"? We have to remember that Paul was writing this to Jews that had accepted Christ but were struggling with the grace that they had experienced and the laws and rituals of their past. He is reminding them that grace is a gift and, if you pay for a gift, then it can no longer be considered a gift.