The Hebrew noun ‘ĕmûnȃ in Habakkuk’s famous statement “but the just shall live by his faith [‘ĕmûnȃ 530]” (2:4), means “faithfulness” as a few translations render it correctly, “live because of his faithfulness” (NET), “live by his faithfulness” (NIV). This word appears about 50 times in the Old Testament and consistently carries the meaning of faithfulness within its contexts. Of course Habakkuk 2:4 is quoted three times in the New Testament, “The just shall live by faith [pistis 4102]” (Rom 1:17; Gal 3:11; Heb 10:38). The Greek noun pistis is used almost 250 times in the NT and is always translated as “faith” except just three places where the context forces otherwise, “the faithfulness [pistis 4102] of God” (Rom 3:3 NKJV), “kindness, goodness, faithfulness [pistis 4102]” (Gal 5:22 NKJV), “showing all good fidelity [pistis 4102]” (Tit 2:10 NKJV). This word pistis also appears about 30 times in the Greek Septuagint, and all but three are rendered into English as “faithfully,” “faithfulness,” “truth,” “trust,” “loyalty,” “reliable,” “steadfast,” or “assuredly.”
Furthermore, Habakkuk wasn’t talking about our faith or faithfulness but God’s, “his faithfulness” (Hab 2:4 NET, NIV). In all three letters he was quoted, “The just shall live by faith [faithfulness]” (Rom 1:17; Gal 3:11; Heb 10:38), it was about God’s faithfulness to the promise He made to Abraham: “For the promise, that he should be the heir of the world, was not to Abraham, or to his seed” (Rom 4:13); “Now to Abraham and his seed were the promises made … that it should make the promise of none effect … God gave it to Abraham by promise” (Gal 3:16,17,18); “For when God made promise to Abraham” (Heb 6:13). About 1,500 years after making a promise to Abraham, God told Habakkuk that He was being faithful to that promise.
Because Abraham didn’t withhold his son from God, “And Abraham stretched forth his hand, and took the knife to slay his son” (Gen 22:10), God promised to not withhold His Son from Abraham, “By myself have I sworn, saith the LORD, for because thou hast done this thing, and hast not withheld thy son, thine only son: That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies; And in thy seed shall all the nations of the earth be blessed” (Gen 22:16-18). That “God will provide himself a lamb” (Gen 22:8), God was faithful to keep that promise, “Behold the Lamb of God” (Jhn 1:29,36).
God had told His people that His love for them was because of the promise He swore to their fathers Abraham, Isaac, and Jacob: “And because he loved thy fathers, therefore he chose their seed after them, and brought thee out in his sight with his mighty power out of Egypt” (Deu 4:37); “But because the LORD loved you, and because he would keep the oath which he had sworn unto your fathers, hath the LORD brought you out with a mighty hand, and redeemed you out of the house of bondmen, from the hand of Pharaoh king of Egypt” (Deu 7:8); “that he may perform the word which the LORD sware unto thy fathers, Abraham, Isaac, and Jacob” (Deu 9:5); “Only the LORD had a delight in thy fathers to love them, and he chose their seed after them, even you above all people, as it is this day” (Deu 10:15). God’s purpose in them was to fulfill the promise He made to their fathers.
Therefore, when Paul wrote, “What advantage then hath the Jew? … shall their unbelief [unfaithfulness] make the faith [faithfulness] of God without effect [katargeō 2673]? (Rom 3:1,3), “Will their unfaithfulness nullify God’s faithfulness?” (NIV), is that God’s people being unfaithful to Him wouldn’t cause Him to be unfaithful to Abraham. If the law of Moses was the intended end, then God’s faithfulness to keep His promise to Abraham is made without effect or unfulfilled, “For if they which are of the law be heirs, faith [faithfulness] is made void, and the promise made of none effect [katargeō 2673]” (Rom 4:14), “And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect [katargeō 2673]. For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.” (Gal 3:17-18).
Finally, the Greek verb zaō in “The just shall live [zaō 2198] by faith [faithfulness],” isn’t our manner of living but eternal life itself. The English “live” when used with an object conveys “manner of living” or “way of living,” but without an object it’s “to remain alive” or “to continue to have life.” Paul used both forms in this one statement, “For if ye live [zaō 2198] after the flesh, ye shall die: but if ye through the Spirit [breath] do mortify the deeds of the body, ye shall live [zaō 2198]” (Rom 8:13). The first is with the object “after the flesh,” expressing a way of living. But the second is without an object indicating “to have life.” Therefore, “The just shall live by faith [faithfulness]” is that the just or righteous obtain eternal life by God’s faithfulness—His faithfulness to Abraham in providing His Son for our sacrifice.