Monday of the third week in Lent: Hebrews 3.
I was particularly struck by this passage:
16 For who, having heard, rebelled? Indeed, was it not all who came out of Egypt, led by Moses? 17 Now with whom was He angry forty years? Was it not with those who sinned, whose corpses fell in the wilderness? 18 And to whom did He swear that they would not enter His rest, but to those who did not obey (ἀπειθήσασιν)? 19 So we see that they could not enter in because of unbelief (ἀπιστίαν).
Not only is this a strong warning about the real possibility of apostasy, but it reasons in a direct line from the disobedience of the Israelites to their unbelief.
This is not a matter of disbelief. There is nothing in the passage to suggest that they resisted the truth of any propositions.
It is also not a matter of lacking an inner spark of faith. YHWH was angry with his people because they were behaving in a way that was disloyal to Him.