Hebrews 6:4 (New Living Translation) For it is impossible to restore to repentance those who were once enlightened—those who have experienced the good things of heaven and shared in the Holy Spirit, 5who have tasted the goodness of the word of God and the power of the age to come—6and who then turn away from God. It is impossible to bring such people to repentance again because they are nailing the Son of God to the cross again by rejecting him, holding him up to public shame.

The passage presents an argument based on a false premise (that a true Christian can fall away) and follows it to its senseless conclusion (that Jesus would have to be sacrificed again and again). The absurdity of the conclusion points up the impossibility of the original assumption. This reasoning is called reductio ad absurdum, in which a premise is disproved by showing that it logically leads to an absurdity. ( http://www.gotquestions.org/Hebrews-6.html )

This passage supports the security of the believer in Christ. It presents the very idea of believers losing salvation as impossible. Many scriptures make it abundantly clear that salvation is eternal (John 10:27-29; Romans 8:35, 38-39; Philippians 1:6; 1 Peter 1:4-5), and Hebrews 6:4-6 confirms that doctrine. (ibid.)

A widespread misinterpretation of this passage has put far too many under condemnation and fear. It’s time to read it in context and see it for the wonderful confirmation it is!