A traditional criticism of the ontological argument is that

The existence of God is the traditional criticism of ontology. Many philosophers argued on this critic like Anselm of Canterbury, Rene Descartes, Gottfried Leibniz, Kurt Gödel, Alvin Plantinga, Mulla Sadra, and Allama Tabatabai. They have raised their ideas and arguments if God does really exist. Actually, Anselm's contemporary was the first critic which using ontology. This was also the first mockery happened in the ontological argument.