Why do critics say that the ontological argument tries to define God into existence?

Critics argue that the ontological argument attempts to "define God into existence" because it treats existence as a defining attribute or property of God’s nature.12

The argument, famously formulated by St. Anselm, posits that God is the greatest conceivable being. Proponents suggest that because a being that exists in reality is greater than one that exists only in the mind, God must necessarily exist to fulfill the definition of being "unsurpassably perfect".31

Critics, most notably Immanuel Kant, contend that this reasoning relies on a logical fallacy: the assumption that existence is a predicate or attribute, similar to color or shape. Kant and other philosophers, such as Gottlob Frege, argued that existence is not a property that can be added to a concept to make it "greater" or more perfect. Instead, they maintain that existence statements do not attribute properties to a subject but rather indicate that the concept has instances in reality. By treating existence as a perfection inherent in the definition of God, critics believe the argument illegitimately bridges the gap between a mental concept and actual reality.1245