What does Hebrews 3:12 mean?

Prior verses quoted from Psalm 95, which warns Israel not to repeat their mistakes in the wilderness. There, Israel lost faith in God. They gave in to fear, and that led to disobedience and stubbornness. They did not "hold fast," and as a result, that generation was not allowed to enter into the Promised Land. God did not send Israel back into slavery in Egypt, but He disciplined the people for their lack of faith.

Here, the application of the Psalm is made explicit. The Israelites' lack of trust caused them to fall away from God, and this resulted in discipline. The author of Hebrews is warning the Jewish Christians who read these words not to make the same mistake.

It's especially important to see the full context of this verse, since it uses two particular Greek words, apistias and apostēnai. These are translated in the ESV as "unbelieving" and "fall away," respectively. Apostēnai is the word from which we get the term "apostasty," which means a rebellion or defiance of authority. It is most frequently used to describe those who completely leave the Christian faith. However, like most such terms, there are varied levels and meanings of "falling away." In this case, the meaning is that of sin and faithlessness, not open rejection of God.

Once again, the context makes it clear that salvation is not at stake. The Promised Land is not a metaphor for heaven—Israel's rescue from Egypt is the symbol of salvation. God's wrath against the Jewish people in the wilderness was not to send them back to Egypt (symbolizing a return to an un-saved state). Rather, it was to deny them the blessings of the promised inheritance. Parts of chapter 4 will further support the idea that the author is speaking of sin in the life of a saved believer, not the potential loss of salvation.