Now that we have looked at Habakkuk 2:4—"the righteous shall live by his faith"— which is key to the New Testament discussion of our justification and salvation, it is time to look at how the most famous quoter of this verse fleshes it out. We are referring, of course, to the Apostle Paul, who uses the verse as the springboard for his fullest presentation of the gospel in Romans and in his response to the Judaizers in Galatians.
In Romans 1:16–17, Paul says that the gospel reveals "the righteousness of God." When the Apostle speaks of revelation, he is typically describing how God more fully discloses under the new covenant something that old covenant believers knew only in part (1 Cor. 2:6–10; Eph. 3:1–6). Although Paul is clear that old covenant saints and new covenant saints are counted righteous in the Lord's sight the same way—by faith in the Messiah alone (Rom. 4:1–12)—new covenant believers understand God's righteousness more fully than old covenant believers did because we live after the cross.
The gospel reveals "the righteousness of God" (Rom. 1:17). In the Old Testament, this phrase denotes several things, including a gift to God's people (Ps. 35:27–28) and His work to save His children in a manner consistent with His own character (Isa. 46:13; Micah 7:9, 18–20). Paul has both senses in mind in Romans 1:16–17, as is clear throughout the rest of his epistle. Romans 3:21–26 and 4:6–8 emphasize the Lord's work to save His people without violating His own perfect standards. The Father gave His Son as a propitiation that satisfied His holy wrath; thus, He does not compromise His just demand that He punish sin when He declares us righteous and forgives us. Romans 4:1–5 and 5:12–21 highlight righteousness as God's gift. The Father justifies us by faith alone on the grounds of the imputed righteousness of Christ. He clothes us with the perfect righteousness of Christ, without which we cannot stand before our Creator unafraid.
Paul's gospel is a legitimate development of the original meaning of Habakkuk 2:4. In Habakkuk's day, many people thought that God had abandoned Israel because evil Babylon was conquering His people. Moreover, from a human perspective, Israel's sin and exile cast doubt on the achievement of the Lord's purposes. Believing God would save Israel and vindicate His servants required absolute reliance on God and His Word alone, not what men could see or do. This is saving faith, which alone can justify us.
Douglas Moo writes, "Justification by faith is the anthropological reflex of [that is, it reflects] Paul's basic conviction that what God has done in Christ for sinful human beings is entirely a matter of grace" (Romans, p. 90). We affirm that faith alone justifies because justification by faith alone is the only doctrine consistent with a salvation entirely grounded in God's grace, based only on His work to save His people.