Hebrews 6:1-8 (ESV)
Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith toward God,  and of instruction about washings, the laying on of hands, the resurrection of the dead, and eternal judgment.  And this we will do if God permits.  For it is impossible to restore again to repentance 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,  if they then fall away, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.  For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God.  But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
“Impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit… if they then fall away.” This has to be the hardest verse in the Bible for those who teach once saved always saved. I don’t know, but people give Lutheran’s a hard time for supposedly ignoring the book of James. It is funny to me, it seems James has become the sum total of the canon for the majority of American Christendom. I think I might just start doing the same thing they do with James, back to them with Hebrews.
That said, I don’t know that I much enjoy this verse. I suppose if there was any reason to suspect the canonicity of Hebrews, it would be this, that it is impossible to restore again to repentance. I say that because I believe all things are possible for God, and I believe that based on books that are certain and have never been questioned for authenticity. I think I may have even seen a few who had fallen away come back to the faith and repentance. To be certain though, it is impossible to look into anyone’s heart. But what reason have I to doubt their confession?
There is though maybe a bit of truth to this. Perhaps this is nothing more than an explanation of the sin against the Holy Spirit. It seems that once people have fallen away from the faith, it is very difficult to bring them back. This is maybe even more true and visited upon the third and fourth generations in those religions, such as Islam, and Mormonism, that have sprouted as thorns and thistles from Christian soil. Wow, is the work hard there. Not impossible, but… well the demons require much prayer before they are exorcised.