These verses give great difficulty to those who believe in an unconditional salvation. It is very clear that anyone who fits this description will not be in God's Kingdom.
If it were not possible for us to fall away, why would Paul even write as he did in I Corinthians 9:27? "But I discipline my body and bring it into subjection, lest, when I have preached to others, I myself should become disqualified [castaway, KJV]." He also warns in Colossians 1:22-23:
In the body of His flesh through death, to present you holy, and blameless, and irreproachable in His sight - if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister.
John W. Ritenbaugh
After Pentecost, Then What?
These verses mean exactly what they say. Every Protestant commentary has trouble with this because, to them, justification and salvation are the same thing, but they are not! Their belief flies in the face of the reality of God's Word. A person who has been justified can fall away, just as verse 6 says. This is reiterated in Hebrews 10:26-31, in what is arguably the strongest language in the entire Bible.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 29)
To receive salvation, a Christian must now live a life of obedience to the law of God. Those who claim that they know Christ and still continue in a life of breaking God's commandments are liars.
Earl L. Henn (1934-1997)
Basic Doctrines: Salvation
Christ's sacrifice applies only once for each person, and if we reject God's grace, it cannot be applied again. This is why willing apostasy is so terrible and why the apostles fought so strongly against heresy in the first century. The eternal lives of thousands of God's people were at stake!
In a more passive way, sin can lead to eternal death by continued neglect. The sinner may know he should repent of sin, but because of lethargy he never bothers to overcome it. He is apathetic; he just does not care. The Laodicean attitude (Revelation 3:15-19) comes dangerously close to this type of sin, and if not repented of, it can lead to the unpardonable sin.
Martin G. Collins
Are Some Sins Worse Than Others?
Other Forerunner Commentary entries containing Hebrews 6:4:
2 Corinthians 13:5
1 John 5:16-17