Habakkuk received his prophetic call at the end of the seventh century BC, but he found God's message hard to believe. After hearing Habakkuk's concern that Judah was going unpunished, our Creator assured the prophet that the nation would be disciplined—and wicked Babylon would be His rod (Hab. 1:1–11). This troubled Habakkuk, who was perplexed that the holy Lord would use an ungodly instrument for His good purposes (1:12–2:1).
God responded by promising that He would reveal all in a vision of Babylon's fall that would come to pass in its appointed time (2:2–5). Until then, righteous people such as Habakkuk had to persevere in living by faith—by maintaining steadfast loyalty to God and obeying Him (v. 4). This reading of Habakkuk emphasizes the righteous person's manner of living, and Paul certainly has this idea in mind when he quotes Habakkuk 2:4 in Romans 1:17. After all, Romans 6–8 details the type of life that justified people live.
To confuse the righteous person's manner of life with his source of life, however, is to deny the gospel. Paul explains this clearly in Romans 1–5, teaching us that the obedience of sinners can never make us righteous in God's sight. Only faith in Christ is able to justify sinners, to render us righteous before the Lord. This concept, too, is taught in Habakkuk 2:4 as Paul quotes it in Romans 1:17b. Due to an ambiguity in the prophet's original words, the Apostle's quotation can also be translated "the one who by faith is righteous will live." This translation accents not the righteous person's manner of life, but its source. The only way for sinners to find life—true, everlasting life at peace with the Creator—is to possess the righteousness that comes by faith. God's gospel reveals His gift of righteousness to His people on account of His saving work in Christ Jesus, but this righteousness is not something we can earn. It is received only by faith in Christ, a faith that rests in Christ alone (Gal. 2:15–16).
The prophet Habakkuk saw that the faithfulness to God that characterizes the life of a righteous individual could never be that person's source of life. He knew God's final salvation of His remnant could not be accomplished through one's works. Thus, Habakkuk was content to put his faith in the Lord and wait for His answer (Hab. 2:1). He understood that he would not be found righteous by doing God's will—as important as that is—rather, Habakkuk knew salvation comes only by believing. Paul develops this truth, showing that the righteous status that guarantees our eternal salvation is possible only through faith, and faith alone.
John Calvin comments on today's passage, "We have now the principal point or the main hinge of the first part of the this Epistle—that we are justified by faith through the mercy of God alone." To truly know the gospel in all of its depth, we must start by being clear on one essential point: eternal life comes only to those who are righteous because they trust in Christ alone. Coming to God through His Son means that we abandon all trust in ourselves and rest only in Jesus.