ontological
Explanation
The comic shows St. Anselm -- the medieval philosopher famous for the Ontological Argument for God's existence -- dressed in full bishop's regalia with a mitre and crosier. He is reciting a version of his famous argument: "By definition, God must be that than which nothing greater can be conceived. From which it follows... that... oh boy, we'll have to cover this up deep."
The caption reads: "Inadvertently, St. Anselm proves that God is nachos."
The joke is a reductio ad absurdum of the Ontological Argument. Anselm's original argument (from his Proslogion, c. 1078) goes roughly: God is defined as "that than which nothing greater can be conceived." Something that exists in reality is greater than something that exists only in the mind. Therefore, God must exist in reality. The comic applies this same logic to nachos: if God is the greatest conceivable thing, and nachos are arguably the greatest conceivable thing (at least to a hungry person), then God must be nachos.
The humor works because it highlights a genuine philosophical criticism of the Ontological Argument -- that the logic of "greatest conceivable being" can be used to "prove" the existence of all sorts of absurd things (the philosopher Gaunilo of Marmoutiers made a similar objection in Anselm's own time, using a "greatest conceivable island"). By substituting nachos for God, the comic makes this abstract philosophical objection viscerally funny. The image of a horrified St. Anselm realizing his own argument has led him to the conclusion that God is nachos -- and that he needs to "cover this up deep" -- adds to the comedy by imagining the historical figure confronting the unintended consequences of his own reasoning.