The Book of Hebrews daunts even the most gifted preachers and scholars. For one thing, we don't know the author. He quotes the Old Testament at length and repeatedly, but his method of interpreting these passages doesn't always make sense to readers. His arguments about angels, Moses, and the temple require more than cursory understanding of the Hebrew Bible.
And then there are the so-called warning passages. It might be hard at first to grasp the significance of the priest Melchizedek, but many Christians viscerally understand the practical importance of these warnings. Can I lose my faith? What if I doubt? Fail to overcome sin?
To answer these questions and more, I turned to the acclaimed scholar Peter O'Brien, professor emeritus at Moore College in Sydney, Australia. Many who have studied Ephesians, Colossians, Philemon, and Philippians have benefited from his rich, insightful, and faithful commentaries. He has also written an immensely helpful commentary on the Letter to the Hebrews. He draws on some of that study to help us understand the famous warning passages in their immediate and canonical context.
Some Reformed teachers find it hard to teach the five warning passages of Hebrews (2:1-4; 3:7-4:13; 5:11-6:12; 10:19-39; 12:14-29). How do we reconcile our theology with what appears to many to be the plain meaning of these passages, that believers can lose their faith?
The warnings of Hebrews have presented many challenges to believers throughout Christian history. And the misapplication of them has caused pastoral problems for Christians of all traditions, including the Reformed.
These warnings have troubled earnest Christians by raising doubts about their assurance of salvation, an assurance that is so clearly affirmed, for example, in Romans 5:1-11 and Romans 8:18-39, and in Jesus's promises for his disciples in John 6:39-40, 44 and John 10:25-30.
Even within Hebrews itself there are powerful words of encouragement and assurance based on God's faithfulness to fulfill his promises to his people (Heb. 2:10; 6:10-20), and so because of the finality of Christ's sacrifice (Heb. 9:11-28; 10:14-18), and his permanent high priesthood by which Jesus is able to save his people completely and eternally since he always lives to intercede for them (7:25; cf. 9:24).
Does Hebrews show us how we might resolve our theological and pastoral difficulties?
A key to addressing the tension between the severe warnings and the seemingly contradictory promises and words of encouragement lies, first, in recognizing the distinction Hebrews makes between "a kind of transitory faith, a form of conversion which, like the seed sown on rocky places [in the parable of the soils, Mark 4], has all the signs of life, but which does not persevere."  Such faith is spurious; by contrast, genuine faith is tied to perseverance.
The conditional sentences of Hebrews 3:6 and 14 fit an evidence-inference category, in which "the observation of a piece of evidence leads the observer to infer a certain logical conclusion." 
We are his [God's] house, if indeed we hold firmly to our confidence and the hope in which we glory (Heb. 3:6).
We have come to share in Christ, if indeed we hold firmly till the end our original conviction (Heb. 3:14).
Accordingly, the author maintains that the listeners' continuance in faith to the end will demonstrate that they are members of God's household, not that they will become this in the future (v. 6). Similarly, holding on to their confidence will reveal the reality that they already share in Christ, not simply that they will share in him on the final day (v. 14).
The listeners' perseverance is the evidence of what has taken place in the past and is an essential ingredient of what it means to be a Christian, a partaker of Christ. So Hebrews "virtually defines true believers as those who hold firmly to the end the confidence they had at first." 
This distinction between genuine and spurious faith is clearly evident elsewhere in Hebrews. This indicates that the author's "word of exhortation" (Heb. 13:22) is addressed to a mixed audience: there are two kinds of soil with dramatically different responses to the frequent showers of God's blessings (Heb. 6:7-8), two kinds of hearts (Heb. 3:12; Heb. 10:22), and a distinction made between "we" who "have faith and are saved," and "the one" who belongs "to those who shrink back and are destroyed" (Heb. 10:38-39). Also the author has concerns for certain individuals within the community who may be in particular danger of apostasy ("any one," "someone": Heb. 3:12, 13; Heb. 4:1, 11; Heb. 6:11-12; Heb. 10:24, 25, 28; Heb. 12:15-16).
Hebrews is not alone in describing true believers as those who hold their confidence firmly to the end. In other New Testament documents there are warnings against or descriptions of spurious faith (Matt. 7:21-23; John 2:23-25; Col. 1:22-23; 1 John 2:19; cf. 2 Pet. 1:10-11). Our Lord's parable of the sower (or the soils) makes a similar point (Mark 4:1-29 and parallels). The initial growth of the seed scattered on the rocky ground and among the thorns appears to all observers, except God himself, to promise the best harvest. But it does not bear fruit. It has the signs of life but does not persevere. This spiritual life proves transitory (Carson 2000: 266).
So how does Hebrews address the audience with its warnings and encouragements?
In view of Hebrews's distinction between true and spurious faith, and its definition of genuine believers as those who hold fast their confession of Jesus Christ to the end, we consider that the images describing the audience in the warnings point to an initial work of grace in the lives of the congregation members.
The author knew that the audience had been exposed to the preaching of the gospel and that God had done a mighty work within the congregation (Heb. 2:1-4). It is evident that some had been truly converted and had genuinely appropriated Christ's saving work for themselves. How many and who they all were, the author does not know exactly. But he addresses the whole congregation on the basis of what he has observed, and urges them to hold firmly to their confession of faith in Christ, their Christian hope without wavering, and their confidence in God (Heb. 3:6, 14; Heb. 4:14; Heb. 6:18; Heb. 10:23).
Significantly, however, even when the author refers to those who commit apostasy he uses the third person plural rather than the second (e.g., "those who have once been enlightened . . . and who have fallen away," Heb. 6:4-6), and does not explicitly identify them with his listeners. Though some are apparently in great danger he does not assert that they have committed apostasy. The warnings, like the divine promises, are intended to prevent this from happening.
The descriptions of the audience in vv. 4-5 ("those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age") point to an initial experience of the gospel. The vivid agricultural imagery of Hebrews 6:7-8, which is integral to the warning of vv. 4-6 and clarifies its meaning, stands between the warning and the expression of confidence in vv. 9-12. It depicts two kinds of responses that can be made to the warning, not simply one, and thus fills out and completes the picture by including both those who do not fall away and those who commit apostasy.
The effects of the rain on each piece of land differ dramatically: in the case of one, the presence of fruitful crops at the end time harvest is evidence of those who had a genuine experience of salvation (vv. 7, 9). But the land that has been well watered and nurtured, and produces only "thorns and thistles" shows that it is worthless, and does not stand the test at the final judgment (vv. 6, 8). The faith of those represented was only transitory (cf. Heb. 10:38-39; Heb. 12:25). They were never true believers, whatever signs of life they may have shown initially.
What is the nature of the sin threatening the community?
From a historical perspective, the nature of the sin referred to involves reverting to Judaism. The listeners are apparently in danger of returning to a reliance on the cultic structures of the old covenant in Judaism.
Although the warning passages of Hebrews describe the sin threatening the community in a number of ways, since there are various facets to it, ultimately it is irreversible apostasy from the living God. It is the utter rejection of an entire position and stance that had once been professed.
This sin is Trinitarian in its scope, for it involves a persistent and culpable refusal to obey the voice of the living God who speaks in his Son and warns from heaven (Heb. 1:1-4; Heb. 12:25). It treats Jesus with utter contempt by crucifying him again, subjecting him to public disgrace (Heb. 6:6), and rejecting his new covenant sacrifice by which the work of atonement was completed (Heb. 10:29). And it arrogantly insults God's gracious Spirit through whom Christ offered himself to God and who applies the definitive forgiveness of sins to believers (Heb. 10:29). The fact that it is willful, persistent, and committed in view of the knowledge of the truth rules out the possibility that it is due to ignorance (Heb. 10:26).
Unlike other sins, offenses, and weaknesses of believers referred to in Hebrews that have been wonderfully atoned for through Jesus' new covenant sacrifice and high priestly ministry, there is no provision for the sin of apostasy. For those who utterly reject God's gracious plan of saving people and bringing them to glory "there remains no more sacrifice for sins" (Heb. 10:26).
In the light of this, the special character of the sin of apostasy must be understood clearly and not confused with other sins and weaknesses of Christians, as has often been the case throughout church history. Since this offense constitutes a total renunciation of everything that is distinctively Christian and which the person had previously professed, it is not the sin of the outsider or the one who is on the edge of church life.
Those who are anxious about having committed this sin, and are troubled that God will not receive them into fellowship with his Son because they believe their sin is too great, are urged by Hebrews to come with boldness to the throne of grace to receive mercy and find grace to help in their time of need (Heb. 2:18; Heb. 4:14-16).
By contrast, apostates defiantly and deliberately reject the Son of God and his salvation, showing neither anxiety nor concern, since they would feel justified in their determined and fixed resolve.
Given Hebrews's distinction between authentic faith as that which perseveres to the end, and spurious faith that may initially show some signs of life but does not endure, the person who commits apostasy is not an authentic Christian and never was one, whatever their first responses to the gospel may have been. And since genuine faith is tied to perseverance that endures to the end, the believer who perseveres in the race marked out for them, with their eyes fixed on Jesus (Heb. 12:1, 2), shows that he or she is a member of God's family and has already been a sharer in Christ.
What are the consequences of committing apostasy?
The consequences of the danger threatening the community to which Hebrews was written have been interpreted in various ways since early times, depending on the nature of the sin that is being described. Those who say this offense falls short of apostasy and is probably some kind of spiritual lethargy that has been manifested in the congregation understand the consequences to be a form of discipline resulting in physical death or the loss of rewards.
But these suggestions do not do justice to the strong language of Hebrews 6:6 or Hebrews 10:26-31. A synthetic examination of the five warning passages shows that the consequences are a "just punishment" (2:2) or no "escape" (v. 3), perishing, missing out on God's promised rest, the tragic loss of their inheritance (Heb. 4:1, 11), the impossibility of being brought back to repentance (Heb. 6:4, 6), which corresponds to the apostate being like land that is "worthless, under a curse, and destined to be burned" (v. 8).
This punishment is not some restorative or disciplinary process but is associated with the severity of the eschatological judgment that will consume God's adversaries. The fourth warning describes the irreversible consequences of apostasy in terms of its severity (it is "terrifying" and "a raging fire") and its finality (it is "inevitable" and "eschatological"). Apostates are cast as God's enemies (v. 27) who are deserving of "far greater punishment" (v. 29) than what the Mosaic law prescribed for rejection of the old covenant, that is, a punishment more severe than merely physical death. Those who shrink back are destroyed which in this setting of final judgment signifies eternal destruction.
The author of Hebrews has not asserted that his listeners have committed apostasy, though he is obviously concerned that some are in significant danger of falling over this precipice. He has warned the whole congregation of the irreversible consequences of apostasy. His warnings, along with other elements in his exhortatory material, together with his doctrinal expositions that provide the presuppositions for the exhortations, are intended to prevent these disastrous consequences from occurring.
In the light of these warnings what does Hebrews exhort them to do?
The listeners are to "hold firmly" to their confession of Christ (Heb. 4:14; Heb. 10:23), and to respond to God and his promises in persevering faith (Heb. 6:12, 15) rather than in unbelief and apostasy that leads to destruction. They are told that they have need of endurance (Heb. 10:36), and so they are to run with perseverance the race that is set before them, fixing their eyes on Jesus, the pioneer and perfecter of faith who endured the cross, and despised its shame in fulfillment of God's will (Heb. 12:1-2).
What assurances do believers have of their eternal salvation?
The encouragements to the members of the congregation to hold firmly to their confession of faith in Christ and to endure patiently whatever trials they may face, are securely based on God's faithfulness to fulfill his stunning promises (cf. Heb. 6:12-20). His purpose is to lead his children to glory, and to that end he has made Jesus, the pioneer of their salvation, perfect through suffering (Heb. 2:10). While the exhortations for them to persevere in the context of trials, persecution, public abuse from opponents, disappointments, and the tendency to lose heart (Heb. 12:5) may seem awesome, even overwhelming, they are not left to their own devices.
Christ's once-for-all perfect offering of himself is utterly acceptable and efficacious; he has blazed the trail for his people into heaven itself, and won for them an eternal redemption. As the Son who lives forever, his priestly ministry on his people's behalf is never ending; he is "able to save completely and eternally those who come to God through him, because he always lives to intercede for them" (Heb. 7:25).
Ultimately, the believer's security rests not with the believer but with the living God. His final promise in the letter, "Never will I leave you, never will I forsake you," is wonderful assurance indeed. So then, "we may say with confidence, 'The Lord is my helper; I will not be afraid, What can mere mortals do to me?'" (Heb. 13:5, 6).
Where would you suggest we turn to learn more about interpreting the warning passages in Hebrews?
D. A. Carson, "Reflections on Assurance," in Still Sovereign: Contemporary Perspectives on Election, Foreknowledge, and Grace, ed. by Thomas R. Schreiner and Bruce A. Ware (Grand Rapids: Baker, 2000), 247-276.
Herbert W. Bateman, ed. Four Views on the Warning Passages in Hebrews (Grand Rapids: Kregel, 2007) particularly the section by Buist M. Fanning, "A Classical Reformed View" (172-219).
And for a more technical treatment, C. Adrian Thomas, A Case for Mixed-Audience with Reference to the Warning Passages in the Book of Hebrews (New York: Lang, 2008).
 D. A. Carson, "Reflections on Assurance," in Still Sovereign: Contemporary Perspectives on Election, Foreknowledge, and Grace, ed. by Thomas R. Schreiner and Bruce A. Ware (Grand Rapids: Baker, 2000), 247-276, esp. 267.
 C. Adrian Thomas, A Case for Mixed-Audience with Reference to the Warning Passages in the Book of Hebrews (New York: Lang, 2008), 184-185. Cf. Carson, "Reflections," 264, 267; and Buist M. Fanning, "A Classical Reformed View," in Four Views on the Warning Passages in Hebrews, ed. by H. W. Bateman (Grand Rapids: Kregel, 2007), 172-219. The majority view, however, understands the conditional sentences in terms of cause and effect.
 Carson, 'Reflections', 267.