Ontology is the study of being, or existence. Ontological arguments are philosophical arguments that God exists, using the concept of ‘being.’ They are usually based on things that we can agree already exist, like the known universe…and extrapolate the necessary existence of God from this knowledge.
The first and most well-known ontological argument was formed by Anselm of Canterbury: that God is “a being than which no greater can be conceived.” Simply put, his argument was that such a being must exist, or it could not be conceived at all.