Hebrews 6:4-6 (NIV)
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.
This verse is controversial and has been used effectively to argue against the security of the believer, the idea that once a person is saved, he cannot lose his salvation. There are so many clear passages of scripture that teach the eternal security of the believer including the magnificent words of Christ in John 10 that it seems foolish to use this difficult passage to trump all the others. If you believe you can lose your salvation, then this verse must not be very comforting since it teaches that you cannot regain your salvation.
I can see how after having stumbled into serious sin that one could wish to be born again...again. That too is certainly impossible according to this verse.
The description given in this verse of the former state of the one having fallen away is unquestionably that of a born again believer, and seemingly a growing one. This is the testimony of a changed life which demonstrates the power of the Spirit of God. What happens if such a person becomes an apostate, living in sin again? The testimony is ruined and Christ has been subjected to public disgrace. There could be no restoration of the previous testimony. And why is that? For the same reason the boy who cried "wolf" is not believed. In the world's view, it is impossible to return to that fresh born-again state. Is salvation lost in this case? No, but rewards are. This is the solemn warming of this passage.