4 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, 5 who have tasted the goodness of the word of God and the power of the age to come— 6 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.
I found this information to be helpful in understanding this passage.
The article describes the two interpretations in more details, but neither of them teach that we can lose our salvation. In summary....
The first interpretation presents unbelievers rejecting Christ and thereby losing their chance of salvation; the second interpretation 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 6:4-6confirms that doctrine.
19 This hope is a strong and trustworthy anchor for our souls. It leads us through the curtain into God’s inner sanctuary. The first part of this verse is very well known (and was the theme verse for our church's day camp program all last summer). And I love its truth! Our hope is secure and totally immovable when it is anchored in God.
But the second part of the verse is also incredibly meaningful - though it is not as obvious to us without the same cultural context as the Jews. The curtain hung between the Holy Place and the Most Holy Place which symbolized the barrier between us and God. Only the high priest could enter the Most Holy Place, and even then, only one a year to stand in God's presence and atone for the sins of the entire nation. But at the death of Jesus this curtain was torn! When we place our faith in Jesus, we are now able to stand in God's presence at all times, not just once a year. What an amazing symbol of the restoration of our relationship with God!
Tomorrow's Bible In a Year Passage: Numbers 19-21, Hebrews 7