What does Hebrews 10:38 mean?

The prior verse began a loose, combined paraphrase of Old Testament Scriptures. One is Isaiah 26:21, the other is Habakkuk 2:3–4. The context of this remark is encouragement, following a dire warning. Earlier, the writer gave a fiery description of God's punishment for those who willfully reject His will (Hebrews 10:26–29). This was followed by words of reassurance, reminding the letter's original readers that they had already endured persecution in the past (Hebrews 10:32–34).

The writer's intent here is to highlight the basic theme carried through the entire letter of Hebrews. This is summarized in verses 35 and 36. God has made it clear the Jesus is His solution for our sin, and with that confidence we ought to "hold fast" to our faith. Those who fall into doubt or disobedience will be subject to consequences (Hebrews 3:12–19), though they cannot lose their eternal salvation (John 10:28–29).

This combination of warning and encouragement fills the gap between two major sections of this letter. Earlier, the writer completed a long, detailed explanation of how the new covenant is superior to the old covenant. Through the rest of the letter, he will expand on the meaning and application of these truths.