What does Hebrews 10:35 mean?

The fundamental message of the book of Hebrews is summarized in verses 35 and 36. Everything else said in this letter is meant to strengthen a Christian's confidence that Jesus Christ is, in fact, the ultimate means of our salvation. Even in the face of persecution and doubt, we have every reason to trust in God and His promises (Hebrews 10:19–20). Those to whom these words are written have, in fact, already survived some level of persecution (Hebrews 10:32). Since the gospel they hold to is so obviously true (Hebrews 10:23), and so incredibly valuable (Hebrews 10:14), and since the consequences of rejecting it are so dire (Hebrews 10:26–29), they ought to "hold fast" to their faith in the face of difficult times.

This confidence is rooted in the truth of the gospel. Hebrews contains extensive evidence that the Old Testament predicted the coming of Christ, as the fulfillment of God's plan for our salvation (Hebrews 8:7–13). At the same time, God has provided examples to warn us about what happens when we fall into doubt and disobedience, such as Israel's suffering in the wilderness (Hebrews 3:12–19). The reward of steadfast faith, then, is not only about gaining something positive; it's also about avoiding the consequences of sin.