The author of Hebrews shows that the words “faith” and “faithfulness” are close for a reason: Because God is faithful, we have a firm ground for our faith.  Thus 10.23: “Let us hold fast the confession of our hope without wavering, for he who promised is faithful.”  And in the great “faith chapter” we read about Sarah, “By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised.”  Sarah is an example for the Hebrews, for the writer has exhorted them to emulate her: “Therefore, holy brothers, you who share in a heavenly calling, consider Jesus, the apostle and high priest of our confession, who was faithful to him who appointed him, just as Moses also was faithful in all God’s house.”

In these cases, when we have faith in God we are reckoning or considering him faithful.  (One could easily use the terms “trust” and “trustworthiness” of God to understand what is being promised and expected.)

The author of Hebrews appeals to (a paraphrase of) Habakkuk 2.4 in exhorting his readers to remain trusting:

But recall the former days when, after you were enlightened, you endured a hard struggle with sufferings, sometimes being publicly exposed to reproach and affliction, and sometimes being partners with those so treated. For you had compassion on those in prison, and you joyfully accepted the plundering of your property, since you knew that you yourselves had a better possession and an abiding one. Therefore do not throw away your confidence, which has a great reward. For you have need of endurance, so that when you have done the will of God you may receive what is promised. For,

“Yet a little while,
and the coming one will come and will not delay;
but my righteous one shall live by faith,
and if he shrinks back,
my soul has no pleasure in him.”

But we are not of those who shrink back and are destroyed, but of those who have faith and preserve their souls.

Habbakuk is a short prophecy that deals with the question of God’s righteousness.  But it also deals with the essential importance of faith in God.  The answer to God’s righteousness (as we see in the conclusion of the book in chapter 3) is found in the fact that his promises are sure and can be trusted no matter what:

I hear, and my body trembles;
my lips quiver at the sound;
rottenness enters into my bones;
my legs tremble beneath me.
Yet I will quietly wait for the day of trouble
to come upon people who invade us.

Though the fig tree should not blossom,
nor fruit be on the vines,
the produce of the olive fail
and the fields yield no food,
the flock be cut off from the fold
and there be no herd in the stalls,
yet I will rejoice in the Lord;
I will take joy in the God of my salvation.
God, the Lord, is my strength;
he makes my feet like the deer’s;
he makes me tread on my high places.

Thus, the statement in Habakkuk 2.4b, “the righteous shall live by his faith,” is exemplified by the ending song in the book.

This is not the only time that Habakkuk 2.4 is used in the New Testament.  Paul’s letter, like Habakkuk is centered on the question of the righteousness and faithfulness of God.  In fact, Paul clearly associates the two descriptions of God’s character as being virtually the same:

What if some were unfaithful? Does their faithlessness nullify the faithfulness of God? By no means! Let God be true though every one were a liar, as it is written,

“That you may be justified in your words,
and prevail when you are judged.”

But if our unrighteousness serves to show the righteousness of God, what shall we say?

Our unfaithfulness demonstrates God’s faithfulness and our unrighteousness demonstrates God’s righteousness.  In this case, Paul is speaking particularly of how Israel’s unbelief and disobedience in crucifying Jesus was in fact how God fulfilled his promise to deal with sin and bring salvation so that, in the Gospel, the story of Jesus’ resurrection, the righteousness of God is publicly declared (Romans 1.1-4, 1.16).

Thus, if God is righteous and faithful, or trustworthy, then the only proper responses is to have faith or trust in him.  Paul makes this thematic for his letter to Romans by using the same word twice as a wordplay:

For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek. For in it the righteousness of God is revealed from faith for faith, as it is written, “The righteous shall live by faith.”

Some tranlsations associate this only with an emphasis on faith (i.e. NIV, “by faith from first to last”).  I think it makes more sense if the wordplay is pointing both to God’s righteousness/faithfulness and our believing response.  Thus, “God’s righteousness is revealed: from [His] faithfulness to [our] faith.”  The same word has both meanings depending on context and here Paul is taking advantage of the ambiguity to show his theme for the letter.

This especially corresponds to the latter part of Romans 3 where we see the dynamic.  Let me represent faithfulness/righteousness/trustworthiness in bold and faith or trust in italics:

“But now God’s righteousness has been manifested apart from the law, although the Law and the Prophets bear witness to it— God’s righteousness through the faithfulness of Jesus Christ for all who believe.”

Excursus: “the faithfulness of Jesus Christ”

Many Bible translations do as the ESV does:

“But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it— the righteousness of God through faith in Jesus Christ for all who believe.”

This is possible, but earlier in chapter 3 the same construction is translated as “the faithfulness of God,” not “faith in God.”  And then again in the next chapter we read of “the faith of Abraham” not “faith in Abraham.”

But what really makes up my mind is the parallel in this paragraph.

But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it— the righteousness of God through the faithfulness of Jesus Christ for all who believe. For there is no distinction: for all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood through faithfulness.

Now, the ESV changes “through faith” or “through faithfulness” to “to be received by faith.”  But Paul could certainly have said that more clearly.  He is talking here about God’s act in history in “condemning sin in the flesh” (Romans 8.3).  This is not something that occurs within a believer’s subjectivity but his objective act in Jesus.  It makes much more sense to say that this was God’s righteousness, providing propitiation.  And that this both demonstrated his faithfulness and was done through Jesus’ faithfulness.

And since Jesus was only faithful by trusting his father, it was also his faith and trust.