According to the Decree on the Sacraments of the Council of Trent, one of the effects of frequent receiving of the Holy Communion is perpetua sanitas mentis, perpetual sanity of mind. The article demonstrates that these words echo a text, which the authors of the decree associated with Saint Cyprian of Carthage († 258), whereas its real author was the monk Ernaldus Bonaevallis († after 1159). It demonstrates that the theme of mental recovery through communion with Christ was present in the theology of the Church Fathers and in the liturgical prayer of the Church, and that the theologians and preachers of the twelfth century, including Ernaldus himself, turned their attention to it. It presents the way in which representatives of High Scholasticism Alexander of Hales († 1245) and Thomas Aquinas († 1274) reflected on sanitas or, more specifically, sanatio mentis. Their theology, and especially the theology of Doctor Angelicus, formed the thinking of the participants of the Council of Trent. It can thus serve as a key for understanding their words.