What does Hebrews 3:15 mean?

Verses 12 through 14 brought a very direct application of Psalm 95 to the discussion. Israel suffered discipline in the wilderness because the people acted in a faithless, stubborn way. The author of Hebrews has warned his readers not to make the same mistake due to their own sin. In fact, he commands Christians to exhort (encourage, challenge) each other to avoid such things. Here, he puts an exclamation point on his use of the Psalm by repeating the first verse quoted, Psalm 95:7.

The use of the term "today," both in Hebrews and in the Old Testament, is meant to be a call for urgency. This is not a topic to be taken lightly, or put off until some other time. The nation of Israel failed to "hold fast" to their trust in God (Hebrews 3:6), and it cost them dearly. They were not cast off by God—just as the Christians this letter is written to are not at risk of losing their salvation—but they were denied their inheritance due to their stubbornness.