This article examines justification by faith in Galatians 2:15–21 in light of the debate between the traditional interpretation of Paul and the New Perspective on Paul (NPP). The debate has focused particularly on the meaning of “works of the law” (erga nomou) and the nature of justification within Paul’s argument. The traditional interpretation, especially within the Reformation tradition, understands justification primarily in forensic terms, namely, God’s act of declaring sinners righteous on the basis of faith in Christ rather than on the basis of works of the law. By contrast, proponents of the NPP, particularly E. P. Sanders, James D. G. Dunn, N. T. Wright, and Scot McKnight, emphasize the socio-historical context of first-century Judaism and the function of the law as identity markers distinguishing the Jewish community from the Gentiles. Through an exegetical reading of Galatians 2:15–21 within the context of Paul’s confrontation with Peter at Antioch, this article argues that the forensic dimension of justification should not be separated from its social and ecclesiological implications. Justification by faith is God’s act of determining a person’s status before him, while simultaneously producing concrete consequences for relationships between Jewish and Gentile believers in Christ. Thus, Galatians 2:15–21 demonstrates the interconnection between justification, faith in Christ, death to the law, life for God, and the formation of God’s people beyond ethnic boundaries.