Starting from the position of Hilary of Poitiers and Augustine of Hippo, the essay examines a specific case of the interaction between reason and faith, that of the ratio applied to the understanding and speculative elaboration of the Trinitarian dimension of God, both as an autonomous ratio and as a ratio aided by faith. The perspective point, therefore, is at one time gnoseological and epistemological. In addition, this paper highlights links with patristic and philosophical sources and with developments in medieval thought.