Some scholars have maintained that Martin Luther (1483-1546) began the disagreements over the Christian's justification by faith or works among commentators on the Epistle to the Galatians. Although acknowledging Luther's pivotal role in this struggle, I argue that by the time of the Council of Trent (1547), the positions of Roman Catholic and Protestant commentators on Galatians were greatly shaped by the disagreements they had received from the commentaries on Galatians of early Christian and medieval authors. Luther and other Protestants used the minority view interpretations from Late Antiquity and the Middle Ages and recast them into a more coherent framework in order to critique the majority view on this Epistle, namely that in Paul's rejection of justification by works of the law, he was merely speaking about Jewish ceremonial rites and not the moral law. Using the arguments of the late antique and medieval minority view, Luther argued that Paul was speaking of the observance of the moral precepts of the law of Moses and, especially, the Ten Commandments. Most Roman Catholic commentators after Luther, however, used the arguments of the majority view from the same eras to provide theological qualification to Paul's aforementioned denial of justification by the works of the law. I document these disagreements among Western commentators from Late Antiquity and the sixteenth century from passages containing Paul's affirmations of justification by faith (2:16, 3:10-13) and Paul's statements about the moral law and eternal life (Gal 5:6, 5:21 6:8). This simultaneously broad and in-depth study demonstrates that by turning the previously debatable question of justification by faith or works of the law into a line distinguishing orthodoxy from heresy, sixteenth-century Protestant and Roman Catholic commentators received these earlier commentarial debates and, unlike their medieval predecessors, formulated polarizing interpretations. Martin Luther made public a centuries-old debate that had been carried out silently at the desks of Western commentators on Galatians. This dissertation argues that the Reformation debates over the meaning of this Epistle's key statements about faith, works and justification had much deeper hitherto unexamined roots in the early and medieval Christian traditions.