The paper addresses the writings of St Augustine of Hippo, thus marking the 1600th anniversary of the anti-Donatist synod held in Carthage and the start of Augustine’s anti-Pelagian argument in 411 AD. The focus is on two orations (Sermo 272 and 227), translated into Slovenian for the first time, which explain the significance of the Eucharist as a mystery of God’s unity and a sacrament of salvation. The contextualisation of their historical and theological backgrounds sheds light on the Donatist and Pelagian controversies, as well as on their implications for Augustine’s thought on the Eucharist from the ecclesiastic and soteriological perspectives. The basic concepts of his sacramental doctrine are defined by the descriptive method. The paper thus concisely confirms the impact of individual controversies on the development of Augustine’s theology, which has exercised a decisive influence on the history of dogmas and practices up to the present.