This paper studies Guardini’s perception of Augustine, especially in his two widely unknown books on Confessions: Die Bekehrung des heiligen Aurelius Augustinus (1935), and Anfang (1944), which are contrasted by the recently published Guardini lectures: Ewigkeit und Geschichte (1955/56 and 1961/62). The author discusses Guardini’s emphasis and fascination with Augustine’s conversion (cf. Conf. VIII), as it was relevant for his existential theology. Additionally, the text explores the Augustinian perception of the phenomenon of conversion by studying the passage from In evangelium Ioannis tractatus 53 (John 12:40), where Augustine delivers a beautiful synthesis about the conversion of the heart through grace: “Conversio gratia est” (Aug., In Io. tr. 53.11). It is hoped this study will evaluate Guardini’s doctrine about the conversion of the heart using the concrete example of Augustine and also replenish and enrich his interpretation by contrasting it to the broader Augustinian corpus.