Okay I am going to side with the conditional salvation argument for a moment.
Hbr 6:4-6 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.
Verses 4 and 5 are unmistakably describing Christians, "enlightened", "tasted of the heavenly gift" and the big one "were made partakers of the Holy Ghost". You also have "tasted the good word of God, and the powers of the world to come", so the writer of Hebrews has gone to great lengths to describe these individuals and left little to the imagination.
So how does one deny the ability for Christians to fall away in light of Hebrews 6:4-6?