There are four major arguments for the existence of God. This is the hardest one to grasp and the easiest one to forget.
This is the only argument of the four which is based strictly on logic, rather than observation. In its simplest form, it boils down to this: because we can imagine God, He must exist.
To go a little deeper, humans have the concept of a “greatest conceivable being.” This is an intuitive thing that exists in our minds. And since existing is greater than not existing, then this thing must exist.
Clearly, this is a logical stretch, and it doesn’t make sense to a lot of people. Most defenses of the ontological argument point this out. In researching this, most explanations of the ontological arguments were a little…apologetic, as if they knew the average person would be highly skeptical of this.
Also, the argument gets reduced a lot, to something like: we can just sense God; or, were have a natural, built-in instinct for God.
Francis Collins, in The Language of God: A Scientist Presents Evidence for Belief, writes about “the fingerprints of God.” He says that humans have a inbuilt instinct to seek the divine, and why would we have this unless this thing existed? This is a form of the ontological argument: since we can conceive of something, it must exist.