Hebrews 6:4

  I am a partaker of the Holy Spirit.
  "For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit." NKJV
  "For it is impossible to bring back to repentance those who were once enlightened--those who have experienced the good things of heaven and shared in the Holy Spirit." JCB

  Paul is talking about believers who reject Jesus by equating this act to nailing Him to the cross once again and holding Him up to public shame (v.6). He makes it very clear that this is a final choice, that there is no coming back...
  I find this difficult to believe.
  Debate has raged around the question about losing our salvation. I don't feel well enough equipped to challenge this idea except by knowing what I know from personal experience.
  In Christ all things are possible!
  Thank God!
  Time and again I've heard stories of people who went to church, who attended Sunday school as children (myself included). They, I, went through a considerable time in the wilderness adrift from faith before they, I, came back to the Lord.
  Or maybe this isn't what Paul is talking about.
  Maybe Paul is speaking about adults making the conscious choice not to believe despite what their senses, their souls, their experiences tell them is true. Maybe Paul is speaking about those who deny the existence of God despite having intimate experiences of His love.
  Even so, I struggle in believing God would turn His back on them forever.

  Maybe that's the difference: I never stopped believing that there was something bigger, something better than the world. For many a dark and lonely year I held on to a vague hope that there was and waited. Did I call this God? I didn't know what to call it. 
  Playing hymns on the organ would fill my heart with a longing for the Something that seemed to be just around the corner. Singing along with love songs playing on the radio became pleas to an unknown Listener...Once in a while, I would catch a glimmer of what was waiting for me.
 
  That's all I want to remember about those years, the moments when the Light penetrated the darkness.
  And when I finally decided to call the wonderful, waited for Something exactly what it was: Jesus.