Hebrews 6:4-6 (KJV)
For
it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put
him to an open shame.
1) "once enlightened" refers to being exposed to the gospel, both hearing it and understanding it, thus enlightened.
2) "tasted the heavenly gift" again refers to hearing and understanding the gospel, but not accepting it or fully accepting it, thus tasted.
3) "partakers of the Holy Ghost" refers to hearing and understanding the inspired word of God, either from the gospel directly by reading God's word, or indirectly from born anew witnesses (the powers of the world to come).
4) If they should reject the good news, it is impossible to bring them to the thresh hold of repentance, unless they change their mind, because there is no other gospel among men whereby they might be saved.