Before giving my opinion on how to interpret warning passages such as Hebrews 10:26-31, I think it would be helpful to clarify the relationship between justification and works. Contrary to popular opinion, justification is not evidenced by the presence of good works in a person's life. Even though people who are justified by faith are characterized by good works thanks to new covenant blessings, justification itself is apart from works, being accomplished by Christ, and thus incapable of being evidenced by good works. At the same time, because of the positive correlation between justification and good works, it is nonetheless tempting to say that good works, while not evidence themselves, are still suggestive of a person being justified. However, this is also problematic because good works are often counterfeited by unjustified people. Therefore, good works provide us with a poor gauge for judging a person's standing with God. In contrast, a person's standing with God should be judged from the profession he makes regarding belief in the Gospel. Does he understand the Gospel? Does he understand the nature of faith? On the other hand, the positive correlation between justification and good works means that a lack of good works is suggestive that a person might not be justified. Despite his profession of faith, maybe he hasn't really understood the Gospel and the nature of faith. Or maybe he does understand them but still hasn't believed. In such a case, it is impossible to make a definitive judgment regarding the person's standing with God. It is in such a case as this that we need to understand warning passages like Hebrews 10:26-31. (Just so there is no confusion, by "lack of good works," I am not suggesting that people who have made a proper profession of faith in the Gospel do not struggle or fall into sin, even begetting sin, but I am saying that there is a problem if such people live a lifestyle characterized by ungodliness and indifference.)
Having said the above, it is crucial to observe that warning passages such as Hebrews 10:26-31 are directed toward people who lack good works. They are not directed toward justified people who are abiding in Christ by faith in the Gospel. As mentioned above, there is a positive correlation between justification and good works, so the problem with the people in the warning passages is that either they are unjustified, meaning that they have not believed the Gospel or that they have believed a false gospel, or they are justified but are currently not believing the Gospel or are not believing it to the extent of being empowered by good works. In the case of justified people who are currently not believing the Gospel, which may mean that they have been deceived by a false gospel or have been overcome by other desires, such people cannot have any assurance of justification while they remain in a state of unbelief. For these people, there is only "a certain terrifying expectation of judgment" (Heb. 10:27), being under the displeasure of God as rebellious children who refuse to grow up or as children who have committed treason like Absalom. They must be threatened in the severest language in hope of "snatching them out of the fire" (Jude 23), for even though they are justified, they are living in a manner characteristic of reprobates. This is why Jesus said to treat such people as publicans and Gentiles (Mat. 18:17), for there is no other way to treat them. The chastisement of God is sure to catch up with them, either in this life or in the afterlife at the judgment seat of Christ where they "will be saved, yet so as through fire" (1 Cor. 3:15).