Hebrews 6:4-6

Hebrews 6:4-6
"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, who have tasted the goodness of the word of God and the power of the age to come--and who then turn away from God. It is impossible to bring such people back to repentance; by rejecting the Son of God, they themselves are nailing him to the cross once again and holding him up to public shame."

This is a much debated passage, and like all scripture, we must take it in light of all of God's Word. We know that it cannot mean that a believer can lose his salvation (see Romans 8:38, 39 and numerous other scriptures). And after all, what does "everlasting" mean? And if it did mean a person can lose his salvation, it would be impossible for him to be saved again.

Some believe that it refers not to genuine believers, but to some who only profess Christianity but do not really possess it.

Charles Ryrie in the Ryrie Study Bible, p. 1858 gives the following further explanation which seems to make the most sense to me:
"Others understand the passage to be a warning to genuine believers to urge them on in Christian growth and maturity. To 'fall away' is impossible (since, according to this view, true believers are eternally secure), but the phrase is placed in the sentence to strengthen the warning. It is similar to saying something like this to a class of students. 'It is impossible for a student, once enrolled in the course, if he turns the clock back (which cannot be done,) to start the course over. Therefore, let all students go on to deeper knowledge.'"

Jesus died once for all, and He will not be crucified again. Those who continue to reject Him will not be saved. Those who trust Jesus will be eternally saved. And while we're here, we don't want to bring shame on His name by our disobedience.