The doctrine of actus purus—typically associated with Aristotle and Aquinas—finds its inchoate metaphysical assumptions worked out in the Nicene controversy. Figures like Marius Victorinus were instrumental in adapting Neoplatonic metaphysics to defend divine simplicity and immutability in a way that upheld Christian Trinitarianism and refuted Arianism. The Council of Nicaea thus became a defining moment, not only for Trinitarian doctrine but also for the metaphysical commitments underlying it. As David Bentley Hart notes, “The advent of Nicene theology began to alter—altogether fundamentally—the conceptual structure of the ancient world.” In line with this shift, Giulio Maspero has recently demonstrated that conciliar Trinitarianism was built upon a conciliar metaphysical foundation, wherein the articulation of homoousios necessitated a reconfiguration of being itself in light of divine transcendence.
This paper argues that the Nicene Creed, in asserting the homoousios of the Son, simultaneously affirms and necessitates the doctrine of actus purus as the only metaphysical framework capable of upholding divine immutability, simplicity, and infinity. This will be demonstrated by showing how Nicene orthodoxy requires a strict distinction between divine pure actuality and creaturely potency, ensuring that no change or composition exists in God, while simultaneously upholding the Nicene affirmation of the Son’s consubstantiality with the Father. Only if God is pure actuality can the Son’s eternal begetting be truly immutable, impassible, and consubstantial with the Father. Any deviation from this distinction, whether through Arian subordinationism or Eunomian univocity, ultimately compromises the transcendence of the Triune God and collapses the creator-creature distinction.