The classic ontological argument, as formulated by Saint Anselm in the early 11th century, comes in the form of a worshipful prayer. Anselm makes it clear from the outset that his belief in God’s existence is already established. The argument serves to expand and clarify that belief. Within this framework, it is best not to understand the ontological argument as an effective apologetic tool.
The wording of this argument can be rather difficult. I will try to explain this argument in a way that is readily understandable. Let’s begin with the assumption that God, if he exists, must be a being so great that no one could possibly imagine any being greater. Let’s say you are sitting in a chair one day, thinking about this being who is so great, you can’t think of anything greater. As you think about such a being, suddenly the thought pops into your mind, and you exclaim, “Hey, I can think of a being greater than the one I’m thinking of now! The one I’m thinking of now only exists in my mind. It would be even greater for such a being to actually exist!”
Now you have thought about two beings in your mind: one that exists and one that does not exist. Obviously the one that exists is greater than the one that does not exist. Which one, then, is God? Well, the one that does exist. Therefore, God exists.
People objected to this argument nearly as soon as it was known (famously, Gaunilo, a contemporary of Anselm). Hopefully, I’ll discuss some of these objections later. But for now I would like to probe a sentence with which Anselm closed in his discussion of how some people claim still that there is no God. Anselm wrote, “What I once believed through your [God’s] grace, I now understand through your illumination, so that even if I did not want to believe that you exist, I could not fail to understand that you exist.”
Here Anselm makes an important statement about what he believes to be a priori knowledge, specifically, that God exists. The human mind, according to Anselm, has innate understanding of God’s existence. Belief in that existence is not a matter of understanding, but of will.