Final answer:
God is considered a noumenon because God is conceived intellectually as the highest being or entity that exists beyond empirical senses, as seen in philosophical definitions and arguments for God's existence by Anselm, Maimonides, and Aquinas.
Step-by-step explanation:
The notion that God is a noumenon stems from a philosophical perspective that sees God as a being or entity beyond our sensory experience and beyond the empirical realm. In traditional metaphysical thought, a noumenon refers to something that is conceived by the intellect although not accessible through the senses or experience.
This is seen in the philosophical and theological arguments of figures like Anselm, who defined God as "a being than which nothing greater can be conceived" and argued for God's existence as a necessary being beyond empirical verification. Similarly, Maimonides grappled with reconciling religious texts with philosophical views on God, proposing that God might not correspond directly to objective evidence and reason. The cosmological and ontological arguments for the existence of God, such as those by Aquinas, consider God as the unmoved mover, the first cause, and a necessary existence that surmounts infinite regress and underlies rationality and design.