What does Hebrews 10:37 mean?

The prior two verses summarize the overall theme of the book of Hebrews. God has made it clear that Jesus Christ is the fulfillment of His plan to save us from our own sin. With that confidence in hand, we ought to "hold fast" to our faith, even in the face of persecution. That summary came as part of the writer's encouragement, reminding his readers—persecuted Jewish Christians of the early church—that they'd already overcome hardships in the past. That encouragement was itself a follow-up to this letter's most strongly-worded warning about the consequences of apostasy and disobedience.

In verses 37 and 38, the writer quotes freely from the Old Testament, paraphrasing and combining statements made in the Scriptures in order to make a concise point. The earlier part of this quotation comes from Isaiah 26:21, the latter from Habakkuk 2:3–4. Both seem to follow the Septuagint, a Greek translation of the Old Testament completed around 200 years prior to the birth of Christ. Neither is quoted absolutely, since the writer is more interested in explaining his point than in making an explicit scriptural reference.

The reference to "the coming one" and the fact that He "will not delay" is part of the writer's encouragement. Past persecutions are over, and the current persecution cannot last forever. Christ will return, at some point in the future, and make all things right.