In his trinitarian theology, Augustine's use of the term principium undergoes a change which has implications for his political theology. In De trinitate 1-7, the application of this concept to the Father first reflects an earlier usage, which follows a Neoplatonic idea of the divine ἀρχή of the One. However, reflection on scripture and religious polemic force a development, and he gradually abandons the term. While he does not abandon the theological idea of the Father's special principium, the relations among the divine persons demand a consideration of the principium of the Son in common with the Father, an idea familiar to the debates on filioque. This Augustinian development is like what Erik Peterson identifies in the Cappadocians: a trinitarian theological development which threatens the monarchical political theology that would otherwise appeal to Christian Neoplatonist thinking. In his trinitarian theology, Augustine's use of the term principium undergoes a change which has implications for his political theology. In De trinitate 1-7, the application of this concept to the Father first reflects an earlier usage, which follows a Neoplatonic idea of the divine ἀρχή of the One. However, reflection on scripture and religious polemic force a development, and he gradually abandons the term. While he does not abandon the theological idea of the Father's special principium, the relations among the divine persons demand a consideration of the principium of the Son in common with the Father, an idea familiar to the debates on filioque. This Augustinian development is like what Erik Peterson identifies in the Cappadocians: a trinitarian theological development which threatens the monarchical political theology that would otherwise appeal to Christian Neoplatonist thinking.