This passage is often used to argue against the doctrine of eternal security. It is argued that those who "fall away" (Heb. 6:6) from the Christian faith will be cursed with eternal hell fire (Heb. 6:7-8). On the other hand, some who believe in eternal security argue that this passage does not describe true Christians or that the danger is only hypothetical. When we examine the details of the passage in context, we find a better interpretation. Those who hold to eternal security should know how to explain these verses to others.
It is very clear that Hebrews is written to Jews who have believed in Christ. There is no indication that the writer switches at any point in his epistle to address unbelievers or mere professing (not possessing) believers (see GraceNotes no. 15, "Interpreting Hebrews: Beginning with the Readers"). More importantly, the immediate context is obviously addressing Christians: They should be teachers (Heb. 5:12); they are babes in their spiritual status (Heb. 5:13); they should go on from their foundational beliefs to maturity (NKJ "perfection," or "completeness," from teleiotes; Heb. 6:1-2); they are qualified by a list of obvious Christian descriptions (Heb. 6:4-5). The purpose of this negative warning is to encourage the readers to go forward in their profession of Christ instead of turning away from it (6:11-12).
The immediate context is neatly bracketed by a concern that the readers are "dull of hearing" (Heb. 5:11) and might become "sluggish" (Heb. 6:12). The author reminds them that they should have grown to be teachers by now (Heb. 5:12). This supports the recurring exhortations in Hebrews to press forward in Christian faith and growth (Heb. 3:6; 4:14; 10:23; 12:1), because these Jewish believers were tempted to return to the sacrificial system of the Mosaic law in order to avoid persecution (cf. Heb. 2:1-4; 3:12; 10:19- 39; 12:1-4). The historical context is probably the persecution of Christians under Roman emperor Nero. Forming the other "bookend" to the passage is Heb. 6:11-12 which is also an exhortation to grow and press on in their faith.
To "fall away" is interpreted by some as apostasy from Christian beliefs or a total denial of the Christian faith. A similar word and thought is seen in 4:11, which refers to the example of the sin of rebellion against the Lord which happened at Kadesh Barnea (cf. Heb. 3:12; Num. 14). The argument and context of Hebrews suggests that this is a falling away from their profession of Christ (Heb. 3:6, 14; 10:23-25; 35-39) which would be the case if they returned to the Mosaic system of animal sacrifices. In the grammar of the original language, falling away is not treated as hypothetical. Other Scriptures show that believers can harden their hearts to the point of abandoning their faith (Luke 8:13; 1 Tim. 1:19; 2 Tim. 2:18).
The first consequence of falling away is that it is impossible to renew those believers to repentance. Those who say this passage teaches that Christians can lose eternal life must admit that it also teaches it is impossible for them to repent so as to be saved again. They would have no second oportunity to be saved.
A better interpretation is that believers who had already repented (changed their minds) about the "dead works" of the Mosaic system (Heb. 6:1; cf. 9:14) can not do that again because they knew better. In the past, they had rejected the Jewish sacrifices and accepted the eternal sacrifice of Jesus Christ. To go back and identify with Judaism is to publicly deny the benefits of Christ's sacrifice and even show implicit agreement that Christ deserved to die, thus the statement in 6:6: "since they crucify again for themselves the Son of God, and put Him to an open shame." With such an attitude it is impossible to bring them back to repentance. These believing readers could make a pivotal decision not to press forward but to deny the provision of Christ's sacrifice and thus forfeit the benefits of professing and growing in Christ. If they do, they can not claim ignorance and start over. Again, this alludes to the pivotal incident at Kadesh Barnea mentioned in 3:7-19 in which those Israelites who decided to turn back were not allowed to enter the Promised Land, though they tried (cf. Num. 14). The author later uses Esau as an example of one who could not have another chance though he "sought it diligently with tears" (Heb. 12:15-17).
The second consequence of falling away is a negative judgment described in Heb. 6:7-8. If God cuts off the opportunity to press on (6:3), the believer will suffer severe consequences. A believer who turns back would be like scorched earth. The imagery of fire unnecessarily leads some to interpret this as hell, but that is a poor conclusion because fire is often used as God's judgment on His people (see GraceNotes no. 34, "Hebrews on Fire"). The believer is compared to the earth which can either bear useful fruit or bear useless thorns; if useless thorns, the earth is "rejected" (NKJ; but adokimos is better translated "not standing the test" and thus "unqualified, worthless"). According to common agricultural practice, earth that bears useless thorns is set on fire to burn the thorns so that the earth might become productive in the future. It is important to note that in the original language there is only one earth, not two, and it (the believer) is not burned, but the thorns (what the believer produces). This judgment of God would be temporal since its goal is the productivity of the judged believer's life (cf. John 15:6).
This passage does not teach that one can lose eternal salvation, nor is it addressing unbelievers or presenting a hypothetical situation. It addresses Hebrew Christians in danger of making a terrible choice to abandon their forward progress in faith to return to Jewish rituals. They would lose forever the progress they would have otherwise made and would suffer God's temporal judgment. This is a good exhortation and warning to Christians today. God wants us to faithfully press forward in our Christian faith. Though our eternal salvation is secure, there are severe consequences if we intentionally turn away from Him and do not go on to maturity. We will not only forfeit the progress we could have made, but face God's fiery chastisement intended to make us more useful in the future.