Habakkuk 2:4, the heart of the entire book of Habakkuk, referring back to the statement of Genesis 15:6,[i] and in light of other Old Testament texts that promise salvation to believers,[ii] states:  “Behold, his soul which is lifted up is not upright in him: but the just shall live by his faith.”[iii]  The great fact that the just shall live by faith was to be engraven plainly upon tablets.[iv]  The ungodly, whether unbelieving Israelites[v] or idolatrous Babylonians,[vi] are proud, their souls lifted up;  in contrast, the people of God, those who are just, shall live by faith.[vii]  Habakkuk sets before Israel the example of Abraham—the patriarch was justified by faith alone, and his faith, because of saving character, produced a life of persevering obedience (cf. Genesis 22).  In such a manner, Habakkuk affirms, the people in his day needed to experience true conversion by faith and evidence the reality of that conversion in a life of faithfulness. A life of open rebellion was unacceptable, but one of mere outward rigorism or moralism would also not suffice, for without a root of faith and a renewed heart, all religious and moral actings were vain (Isaiah 1:10-15; Hebrews 11:6).  The word faith [viii] (‘emunah) in the verse, a noun related to the verb believe in Genesis 15:6, means in Habakkuk 2:4 a steadfast trust which results in faithfulness, combining the ideas of faith and of faithfulness that flows from it.[ix]  It is used for stedfastness and steadiness,[x] God’s truthful faithfulness,[xi] human faith, truthfulness, and faithfulness,[xii] and what is true and faithful in itself.[xiii]  Other words in the ‘aman word group, that of belief/faith/faithfulness,[xiv] mean faithfulness,[xv] verily, truly, indeed,[xvi] trusting, faithfulness,[xvii] faith, support,[xviii] constant,[xix] and firmness, faithfulness, truth.[xx]  Thus, as Genesis 15:6 indicates that believers are righteous, Habakkuk 2:4 indicates that those who are just are those who live by faith—and faithfulness is impossible without faith, for those who have, through the instrumentality of faith, embraced Jehovah as their own God and trusted in His promise of redemption through the Seed, will also characteristically trust in God and live their lives as the people of God out of the faith that is the fundamental or radical root of their spiritual life. Righteousness, life, and faith, in both their earthly “already” and their eschatological “not yet,” are indissoluably connected.

The notions of “faith” and “faithfulness” lie close to one another, and are not uncommonly expressed by a single term (so pi÷stiß, fides, faith). . . . “[F]aith,” in its active sense . . . occurs in the Authorized Version of the Old Testament [in] Deuteronomy 32:20 where it represents the Hebrew NUmEa, and Habakkuk 2:4 where it stands for the Hebrew ‘emunah; and it . . . [is] really demanded in . . . Habakkuk 2:4. The very point of this passage . . . is the sharp contrast which is drawn between arrogant self-sufficiency and faithful dependence on God. The purpose of the verse is to give a reply to the prophet’s inquiry as to God’s righteous dealings with the Chaldæans. Since it is by faith that the righteous man lives, the arrogant Chaldæan, whose soul is puffed up and not straight within him, cannot but be destined to destruction. The whole drift of the broader context bears out this meaning; for throughout this prophecy the Chaldæan is ever exhibited as the type of insolent self-assertion (Habakkuk 1:7, 11, 16), in contrast with which the righteous appear, certainly not as men of integrity and steadfast faithfulness, but as men who look in faith to God and trustingly depend upon His arm. The obvious reminiscence of Genesis 15:6 throws its weight into the same scale, to which may be added the consent of the Jewish expositors of the passage. Here we have, therefore, thrown into a clear light the contrasting characteristics of the wicked, typified by the Chaldæan, and of the righteous: of the one the fundamental trait is self-sufficiency; of the other, faith. This faith, which forms the distinctive feature of the righteous man, and by which he obtains life, is obviously no mere assent. It is a profound and abiding disposition, an ingrained attitude of mind and heart towards God which affects and gives character to all the activities. Here . . . the term . . . in the Old Testament . . . rises to the full height of its most pregnant meaning. (“The Biblical Doctrine of Faith,” Warfield, in Biblical Doctrines, vol. 2 of Works).

In both the Old and New Testament, “[t]he trusting man (NyImSaAm = pisteu/wn) is also the faithful man (NDmTa‰n = pisto/ß.”(pg. 198, Theological Dictionary of the New Testament, vol. 6, Kittel).