do not drift alone

The warning in Hebrews 3 is not aimed at outsiders. Heiser keeps pressing this: the writer addresses them as brothers, and he is talking to brothers about the danger of an unbelieving heart. Whatever this is, it is not a danger that only other people have.
Then look at the safeguard he gives. Not try harder. Not examine yourself more. "Exhort one another every day, as long as it is called 'today.'" The remedy for a heart that hardens quietly is other people, frequently. Drift happens in isolation, so the antidote is prescribed as a daily, shared habit.
Scripture links:
- Hebrews 3:12-13 - "Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called 'today,' that none of you may be hardened by the deceitfulness of sin."
- Hebrews 3:14 - "For we have come to share in Christ, if indeed we hold our original confidence firm to the end."
Nobody notices their own hardening. That is exactly why he tells you not to do this alone.