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)
Those who live their lives in union with God in this age will take part in the resurrection to eternal life. However, those who have tasted what God offers and rejected it—"those who have done evil" (John 5:29)—will be resurrected to face their Judge, and then they will be cast into the Lake of Fire and die the second death (see Revelation 20:12-15; 21:8).
Verse 8 then relates that the fate of such people is to be burned. They will have died once already, yet that first death will not satisfy the penalty for sin. Death by old age, disease, accident, or violence (including suicide) does not pay the death penalty for sin. Only a life taken in judgment for sin satisfies the debt.
Christ's sacrifice is one such payment. However, if an individual will not allow Christ's blood to pay that debt, the only recourse is for his life to be taken in payment for his sin. If he is determined to live in opposition to God, unconcerned about obeying God's commands, that person would be miserable living forever anyway. He will not be given the gift of eternal life in a state of mental or physical torment.
Instead, John 5:29 speaks of a “resurrection of condemnation.” Paul says there will be “a resurrection of the dead, both of the just and the unjust” (Acts 24:15). Similarly, Daniel 12:2 mentions those who “shall awake . . . to shame and everlasting contempt.” Anyone remaining in such opposition to God will be resurrected to physical life, judgment will be passed, his body will be burned in payment of his debt, and he will cease to exist. If he is even remembered, the memory will be contemptible.
This is why the second death continues as a theme throughout Scripture, always in the background but rarely mentioned. It is the final event for those who choose to remain in opposition to God after being given the opportunity to know Him. Paul describes this in Hebrews 10:26-27: “For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries.”
We who are in Christ have eternal life. We will still undergo a physical death, but eternal life is ours—and ours to lose. When we survey the warnings given in the New Testament, they are largely not about a sudden, dramatic turn away from God. Rather, they are about smaller things—little decisions of death that require time to bear evil fruit.
So there are warnings about false teachers, who will, over time, damage the faith on which we stand. The writers warn about deception, the cares of this life, and the enticements of this world. They caution us about growing weary and apathetic and neglecting this great salvation. They admonish us against letting the wrong attitudes take root. The dangers are subtle and incremental, but each one has the potential to lead us slowly away from God.
While any one thing may not seem critical today, the problem is what is produced tomorrow—which we often cannot foresee. Carelessness takes us to where our hearts no longer care about overcoming, and we become hostile toward God and the things of God. It opens us to the same lie that Eve fell for: that we can do as we please and continue living. The fact is, though, the spiritually dead do not know they are dead—they believe they are alive.
It is unlikely that anyone sets out to choose the second death. Instead, it is chosen incrementally, with all the little choices over time creating a character that is set and unchangeable. That character will either be intent on overcoming, on hearing Christ's voice, and on trusting in God, or set in opposition to God and His law (Romans 8:7) and thus rejecting life. The choice is ours.
David C. Grabbe
What Is the Second Death?
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