Everything about the Warning Passages in Hebrews hinges upon the audience:
Who are they? Are they believers or not?
I begin with this observation: in the history of the Church many have made a distinction between a genuine believer and a nominal believer. I find such categories useful in some contexts. The issue in reading Hebrews is whether or not the author uses such a category to explain his audience. And, this means we have to ask if it is meaningful to use in Hebrews or an imposition, designed perhaps to protect our theology.
Again, there are plenty of things to consider.
First, the author often includes himself with the audience by using the term “we.” 2:1-4; 3:14; 4:1, 11, 14-16; 6:1; 10:19; 12:1-3, 25-29.
Second, the author calls his audience “brothers.” 3:1, 12; 10:19; 13:22. Perhaps 3:1 needs to be quoted: “holy brothers who share in the heavenly calling.”
At 2:11-17 we have the following thread about what “brother” means: “For this reason Jesus is not ashamed to call them brothers [and sisters], 12 saying, I will proclaim your name to my brothers [and sisters], in the midst of the congregation I will praise you.17 Therefore he had to become like his brothers [and sisters] in every respect, so that he might be a merciful and faithful high priest in the service of God, to make a sacrifice of atonement for the sins of the people.”
Third, at 4:3 he calls his audience “believers.” This text is not distinguishing genuine from false, but believers from non-believers. Believers, it says, enter into the rest. [Yes, it needs to be noted: a believer who enters the rest perseveres. But, this does not mean that those who do not persevere were not believers, but that those who do not persevere will not enter the rest.]
Fourth, sometimes the author sees his audience as “you.” This suggests he thinks some of them will not make it. See 3:12; 5:11; 12:18-24.
Fifth, 10:29 needs to be read carefully: “How much worse punishment do you think will be deserved by those who have spurned the Son of God, profaned the blood of the covenant by which they were sanctified, and outraged the Spirit of grace?” Here the “you” have spurned the Son of God, and profaned the blood, and were (already) sanctified by the blood, and are outraging the Spirit.
Put together, this all indicates a full Christian experience: conversion, gifts and manifestations of the Holy Spirit, the work of the death of Christ, and a Christian community commitment.
Seventh, now briefly on 6:4-6: the author claims that those who have reached a certain level and turn back cannot be restored unto repentance. (This is a singular comment; it is grave.)
Enlightened: see 10:32. An early Christian conversion term.
Tasted…: see 2:9; 6:4, 5. This does not mean “taste” as in dabble, but is a metaphor for “experience.” See at 2:9 — one does not merely “dabble” in death; it means to die.
Partaken in the Spirit: refers to early Christian experience of the Holy Spirit.
Tasted Word… again, experienced the powers of God’s Word.
Again, these verses put it all together: a full Christian experience.
Here’s my summary: indeed, the author sees his audience as mixed. Mixed, in the sense of those who will persevere and those who will not. Not mixed in the sense of frauds and genuine. There is no suggestion in the book of the latter category, but plenty of the former. There is all kinds of evidence that he thought some would persevere and some would not; he never suggests those who do not persevere are frauds. There is a big difference.
My conclusion is this: the author of Hebrews saw his audience as believers but knew that some would fall away, or had fallen away, or might fall away. For those who did, there would be no final rest. The implication is that a believer can fall away.