It is NOT about Losing your salvation, but about Forfeiting it, the scripture clearly teaches BOTH the sovereignty of God AND the free will of men. To deny this is irrational, and must be caused by something other than reason based on scriptural precepts.
Hebrews 6:4-6,  "For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit,  and have tasted the good scripture and the powers of the age to come,  and then have fallen away, it is impossible to renew them again to repentance, since they crucify to themselves the Son of God, and put Him to open shame.
This is one of the classic proof texts for forfeiting one’s salvation. Most Calvinistic theologians have made this passage fit into their theology by claiming that the passage does not refer to believers. There are, however, some significant problems with attempting to do this. First, the text clearly says, “it is impossible to renew them again to repentance”, the “them” referring to those who “have fallen away”. If the Calvinistic theologians interpretation is correct, then the text says that it is impossible for those who never actually became believers (repented, believed and thereby received the Holy Spirit), to repent and become believers. This seems to be inconsistent with God’s mercy (2 Pet. 3:9; Matt. 11:28). It also seems to contradict the entire 18th chapter of Ezekiel, particularly verses 21-23. In addition, Jesus is clearly the object of the “elementary teachings” (verses 5:11 & 6:1), so saying that this passage refers to unbelieving Jews really stretches the concept of consistent interpretation.