Immanuel Kant rejected the existence of God on empirical grounds, but he then tries to double down by attacking an a priori (theoretical) argument by arguing against the necessity of existence. He argues that an absolute necessity of a statement does not form the absolute necessity of a thing. For example, to describe triangles as having three sides does not mean that triangles necessarily have to be real or exist. Thus, to describe a maximally greatest being does not mean that such a being necessarily has to be real or exist. When Kant argues this way, he is rejecting the use of existence as general predication. He does not think that people can say that God is exist. In other words, exist does not stand in a general predicated position by telling something about God or describing an attribute like in the statement the man is tall.
While I understand the reasoning here, and maybe I’ll grant that to use “exists” as predication does not make sense for things that we know are real, like triangles or a man, but I argue that when we are talking about possibly existing things, like a maximally greatest being, it is intelligible to ask whether existence is one of its ontological attributes. Thus, existence can be used in a general predicated way. In other words, an attribute of the maximally greatest being is that it exists.