Which philosopher argued that existence is a defining predicate of God?

The assertion that existence is a defining predicate of God is most closely associated with St. Anselm. Anselm is known for his ontological argument for the existence of God, which asserts that God, being defined as the greatest conceivable being, must exist in reality because existence is a perfection. If God only existed in the mind and not in reality, then it would be possible to conceive of a greater being—one that does exist, thus contradicting the definition of God as the greatest being.

Rudolf Otto focused more on the nature of religious experience and the ineffable qualities of the divine rather than the concept of existence as a predicate of God. René Descartes, while he did argue for the existence of God, framed his arguments around the idea of clear and distinct perceptions and the necessity of a perfect being but did not claim that existence is a defining predicate in the way Anselm did. Thomas Aquinas focused on proving God's existence through empirical observation and the nature of causality, which does not directly equate existence with a defining predicate.

Thus, St. Anselm's formulation that existence is an integral aspect of God's nature establishes him as the philosopher associated with this claim.