This dissertation is a study of St. Augustine's commentary on Psalm 118 (119). One of the latest works of Augustine, Enarratio in Psalmum 118 can be considered as the crowning Augustine's commentaries on the Psalter. The first part of the dissertation examines the historical and literary context in which Augustine wrote his expositions on the Psalms, to which Erasmus gave the title of Enarrationes in Psalmos. The second part studies Augustine's Enarratio in Psalmum 118 in the context of patristic exegesis on Psalm 118 and underscores the originality of Augustine's exegesis, the date and audience of the commentary, and the literary and theological structure of the exposition as a whole. In particular, the study highlights the use of the notion of the Totus Christus, through which Augustine develops the description of the Church as a people walking on the way of the Lord that is Christ, the end of law. Augustine's commentary on this Psalm of the law thus becomes the narratio of a Church graced with God's guidance and mercy in Christ.