Every proper name represents an object of a certain kind and belongs to one or more classes of objects that can be defined. Thomas Aquinas can be classified as a medieval theologian, philosopher, and a human being. “Zeus” and “Apollo” are proper names that designate unique objects, but they are also objects that belong to the class known as gods worshiped by the ancient Greeks, a class that includes Aphrodite and Pallas Athena as well.
When we use “God” as a proper name, we are using it to designate an object that is not only unique, as every other singular individual is, but one that is also unclassifiable.
– How to Think about God by Mortimer J. Adler