This study seeks to recover the richness of Augustine's ecclesiology by exploring the development of his thought on the Church as a great 'mystery' and 'sacrament' (magnum sacramentum). It contributes to the scholarship that documents the shift from Augustine's earlier, more Platonizing views to his mature Biblical and sacramental theology, particularly after his reading of Paul in the 390s. Chapter 1 examines the distinction between the Latin terms mysterium and sacramentum for the translation of the Biblical μυστήριον. Augustine inherits this distinction from predecessors such as Cyprian, Hilary, and Ambrose, and applies it to the Church in order to unite the 'invisible' and 'visible' aspects of the one mystery. For Augustine, the Church resists reduction to an 'inner, invisible' reality, for her 'outer, visible' condition in history is intrinsic to the mystery revealed by Scripture.  Chapters 2 and 3 trace the development of Augustine's ecclesiology through his use of two key themes: 1) the body of Christ, and 2) the bride of Christ. Augustine's early works, such as the Cassiciacum dialogues, reveal the influence of Neoplatonism upon his thought, with an emphasis on the ascent of the Soul that yields vision. After the Biblical shift in Augustine's thought, he reconfigures the Plotinian ascent according to the paschal mystery, such that vision is subordinated to the charity mediated through the Church's sacramental life. As the body and bride of Christ on pilgrimage, the Church undergoes a process of transformation and growth in history by participation in a sacramental economy. Chapter 4 traces the development of Augustine's theology of sacrifice from an individualistic, spiritual sacrifice of the mind to a communal offering celebrated visibly at the Eucharistic altar. In her celebration of the Eucharistic sacrifice, the Church learns to offer herself as a sacrifice in conformation to Christ, and so the Church is herself a 'sacrament' (sacramentum), a sacred sign, of the invisible sacrifice of the 'whole Christ' (totus Christus) offered in the daily sacrifice of Christians. Augustine's mature Eucharistic ecclesiology serves as a patristic resource for Lumen Gentium's notion of the Church as the 'universal sacrament of salvation.'