The phrase, "The just shall live by faith", is quoted three times in the New Testament.
Romans 1:17, Galatians 3:11, & in Hebrews 10:38. One of the many reasons people believe that Paul wrote the book of Hebrews is because of this quote. Paul most likely writes all three of these books, but he takes two words from this verse, (The just shall live by faith) in Habakkuk 2:4, he builds his whole teaching on this verse. It takes Paul three entire books to explain this one verse and this is the break down.
The book of Romans teaches us how we can be justified. The book of Galatians teaches us how we ought to live. And the Book of Hebrews teaches us that what faith is. So this is the break down of all three books combined:
Roman = The Just
Galatians = Shall Live
Hebrews = By Faith
Three entire books explaining six little words. Pretty cool isn't it?