Hebrews is one of the three New Testament commentaries on a single Old Testament verse:
The just shall live by his faith. (Hab 2:4b KJV)
This is the verse that struck a fire in the heart of Martin Luther, and began the Protestant Reformation 450 years ago. This verse opened the eyes of Augustine, and helped him to become a mighty man of faith, and it is still striking fire in many hearts today. It is expanded and amplified in Romans, Ephesians, and Hebrews Each of these Epistles emphasizes a different aspect of that statement.
The book of Romans talks about the just — the justified — those who have been accepted as righteous in Jesus Christ. The just shall live by faith. The book of Ephesians emphasized the words "shall live," and it tells us about life as a justified person — the walk in the Spirit, the life in Jesus, the life of Christ in us — the just shall live by faith. And finally, the book of Hebrews takes up the last two words,
by faith, and it shows us how to lay hold of the life by which we are justified.