For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. (Hebrews 6:4-6).
Does that mean a saved person can fall away and never be able to be saved again? No.
Charles Spurgeon did the best job I've read on the clarity of the Hebrews verse. See if you agree (or don't ;)
You know how many passages there are in which it is positively asserted that if a child of God did deliberately and totally apostatize, his restoration would be utterly impossible—not difficult, but impossible. This is one of the greatest proofs of the doctrine of the final perseverance of the saints, since there is no man in a condition in which it is impossible to save him, and yet any man would be in such a state if he apostatized. Therefore true believers shall not apostatize, but shall stand fast, and shall be kept even to the end. Yet, could they totally apostatize, they could never be restored again: the greatest remedy having already failed, there would remain no other. C.H. Spurgeon