28 It is through his many extant Sermons and Epistles that he principally lays claim to the title of Doctor. The Sermons cover a great variety of subjects, nearly all of which have some connection with the liturgical cycle. In all these writings he is not just the exegete elucidating a Book of Sacred Scripture, not just the theologian at pains to investigate some divinely revealed truth. He is the saintly exponent of the Christian mysteries. He explains them with clarity and with a wealth of detail, in accordance with the faith of the councils, the Fathers, and the popes who preceded him.
Source: Aeterna Dei Sapientia (Vatican.va)