This study traces the first steps of how divine simplicity entered into Christian Trinitarian discourse. It is the burden of this thesis to demonstrate that divine simplicity emerged in the ante-Nicene period with a distinctive status quaestionis concerning (a) the meaning of the doctrine, and (b) its function in reflections on the Father-Son relation. The first part argues that simplicity emerged in the ante-Nicene period with two possible trajectories of interpretation, anticipated by Plato’s Republic and Phaedo respectively. In the apologists, divine simplicity emerged as a purely metaphysical doctrine. However, a richer interpretation of the doctrine is also available in ante-Nicene theology, as exemplified in Origen’s understanding of divine simplicity as a metaphysical-ethical synthesis, meaning that (a) God’s nature is perfectly incorruptible, and (b) God’s character is perfectly free from contradictions. The second part argues that divine simplicity acquired a role in ante-Nicene reflections on the Father-Son relation within two significant ante-Nicene contexts: (a) polemic against Valentinian emanation (prolatio/probolē) and (b) polemic against Monarchianism. The genius of Origen is to utilise divine simplicity for avoiding the Monarchian identification between the Father and Son on the one hand, and the Valentinian separation between the Father and Son on the other. Consequently, we find the surprising conclusion that divine simplicity serves as a principle of differentiation as well as unity between the Father and Son. This thesis raises new questions for both modern theologians and patristic specialists. For modern theologians, the ante-Nicene developments suggest the Son’s generation as a fruitful site for further analysis on the relation between divine simplicity and Trinitarian theology. For patristic specialists, ante-Nicene developments highlight the need to account for the transition from the ante-Nicene to the post-Nicene status quaestionis: how did divine simplicity change from being attributed to the Father (ante-Nicene) to being attributed to the divine essence (post-Nicene)?