6:4 impossible. This passage (Hebrews 6:4-6) has been very controversial, the argument having to do with whether or not it teaches that a true Christian can lose his salvation. One very clear teaching that emerges, however, is that a person can only be saved once. If a true Christian actually could fall away and be lost again, he would be eternally lost; it would be impossible to bring him again to repentance.
6:5 world to come. In context, the author of Hebrews was warning the Jews who had professed faith in Christ not to relapse back into Jewish legalism and ritualism, but rather to go on to full maturity in Christ. The question is whether or not they were actually born again, truly believing on Christ. Could professing Christians be enlightened partakers of the Holy Spirit, having tasted of the heavenly gift and the Word of God, as well as the energizing knowledge of the world to come, without actually being born again Christians? These criteria all certainly apply to real Christian believers, but they also seem to apply, in some measure at least, to the considerable number of men who at one time were members of evangelical churches and later became apostates from the faith. In fact, it often happens that the most vigorous opponents of true Biblical Christianity are men who once were fundamentalist Christians but later—through their studies in evolutionary science, humanistic philosophy or Biblical criticism—came to deny the faith they once had espoused. Such men never return, for they have already understood and rejected all the evidences in both the Word and the world that the gospel is true, and there is nothing more that can be said to win them back. “If we sin wilfully after we have received the knowledge of the truth, there remaineth no more sacrifice for sins” (Hebrews 10:26). It is therefore vitally important that each professing Christian “examine yourselves, whether ye be in the faith; prove your own selves” (II Corinthians 13:5). “Give diligence to make your calling and election sure: for if ye do these things, ye shall never fall” (II Peter 1:10).
6:6 If they shall fall away. “If they shall fall away” from the true faith, having once fully understood it (“been enlightened,” having been made “partakers of the Holy Ghost,” etc.), then they can never return. “My Spirit shall not always strive with man” (Genesis 6:3). However, this seems to be a hypothetical situation—“if” they fall away. The question remains: can they fall away? The security of true believers in Christ is clearly taught in numerous other passages (John 10:27-29; Romans 8:35-39; Ephesians 1:11-14; I John 5:11-13; etc.), and all these prove that genuine Christians cannot fall away, for Christ Himself will keep them from it. Just as a babe, once born, cannot be unborn, so one who is “born again” into God’s family can never be not born-again. One is saved from hell when he truly accepts Christ, so he can never end up in hell; if he does, he certainly was not saved from it. The very concept of salvation would become meaningless. Nevertheless, the warning remains very clear. Many who appear to be true Christians and who may even believe themselves to be true Christians, can and do fall away and come to deny and oppose the faith they once thought they believed. They could not truly have believed it, however, or they would never have allowed doubts to come in and supersede the overwhelming evidences of its truth. “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us” (I John 2:19).