Your Question: Are backsliders lost forever, as Hebrews 6 seems to teach?
Hebrews 6, like any other passage of Scripture, is to be read in conjunction with other Scriptures to ensure we have the proper meaning of what the author is saying. Scripture teaches us that God alone saves us from our sin and His wrath (Ephesians 2:1-9; Romans 3:21-31). Scripture teaches us that God, out of His good pleasure and eternal purpose, has chosen those to be His through election (Ephesians 1:3-14). Since God purposed His children to salvation from all eternity, He is faithful to carry it on to completion and that He preserves us in faith to the very end (Romans 8:28-29; Philippians 1:6). It is important to hold these truths in mind as we read and study Hebrews 6.
Fallen from grace
At its face, Hebrews 6:4 seems to be saying that those who were saved have fallen from grace and cannot be restored. Here are the words of Scripture: “For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.” The term “back-sliding” is a popular description of a person who at one time seemed so passionate about Jesus and now has “fallen away.” What are we to do with this passage when we know from the counsel of Scripture that nothing can take us away from God (John 10; Romans 8:31-39)?
First, we must observe that this entire statement is used as a warning against the hypothetical situation. Believers and non-believers need warnings and corrections to continue in what they have learned so that God may continue to work in the life of the believer and that the non-believer may have a chance to respond to God’s call. Verse 9 gives us a clue that this warning against “falling away” is in hypothetical terms. It is almost as if the author of Hebrews is saying, “but what I warn against is not the case with you because I have seen what God has done in you and through you” (my paraphrase). Because God’s work in their lives seems so strong (even though they’ve not yet moved beyond the elementary teachings), it seems unreasonable for the author to the Hebrews to conclude that they were anything but saved.
What type of person
Second, we have to take into consideration what type of person the author is describing. He is not describing a person God foreknew and predestined to salvation. He is not talking about a person like Peter whom Satan desired to sift like wheat and yet had Jesus praying protection over him. Rather, he is talking about a person who experiences the benefits in the life of the church and turns away. The classic example of such a person is Judas Iscariot who sat at the feet of Jesus for 3 years who never truly trusted Jesus in the first place. It is possible for unbelievers to learn Scriptural truth, make professions of faith, and experience the wonder of the Holy Spirit.
We have all seen our friends and family turn toward things of religion. They desire to visit a worship service, and may serve in some charitable function. Perhaps they sign up for a bible study or small group. Perhaps they have a religious experience where they felt the presence of God, and attest to it. Perhaps they evangelize others. In seeing this, we get excited that they have embraced the faith passed down from the saints. We celebrate that they may have turned a corner. But then they stop coming to worship. They turn away, abruptly or gradually, and they seem inoculated to the truth of God. They heard it, “tried it,” and realized it is not for them. The truth is that the wheat and weeds grow up together in the outward visible church and many times cannot be distinguished by the eyes of men. Only God knows those who are His. People who fall away and are not restored were never Christians at all. This passage speaks of a person who heard the Gospel and learned the great truths of God, but eventually the seed planted never took root. In fact, when you speak to the person who fell away, you get the sense that a hardening of the person’s heart took place in the process. God was gracious enough to allow them to experience Him even though they never truly believed.
Is there still hope?
At this point, I have not answered the question, but have given only background. Is there still hope for a person who turns his back on God? From the human standpoint, we are to live our lives in pursuit of these people that they may be restored. Pray for them. Love them. Have conversations with them. Paul commands that the Corinthian church discipline a person who is caught in gross sin so that he may come to his senses. In some cases, there may be opportunities in which the church needs to be blunt and lovingly discipline the person so that he may be fully aware of his sin (1 Corinthians 5). From there, the rest is in God’s hands. There are multitudes of stories of people who confessed belief in Jesus, turned away, and then were lovingly restored and serve Jesus. Since we do not know the inwards hearts of men, we are to do everything we can to encourage them to continuously seek the Lord while He may be found.