Perhaps one of the most complicated yet compelling arguments for God is the ontological argument. It states that for God to be God, He must be a real entity, because if He wasn’t, then a greater possible being could be envisioned, which is contradictory to the idea of God.
The argument follows as such:
God is defined as a maximally great being. A being whose "non-existence" is logically impossible is greater than a being whose "non-existence" is logically possible. Therefore, if God only exists as an idea in the mind but does not necessarily exist in reality, then we are able to imagine something greater than God. As God is defined as a "maximally great" being, we cannot imagine something greater than God. Thus, it follows that if God exists as an idea, then He necessarily exists in reality in order to fulfil His very definition.
A common objection to the ontological argument is that it’s unduly complicated and more akin to deceptive word play than an honest argument. While it is not an easy argument to grasp on first reading, it adheres to logical principles throughout, as long as one key crutch of the argument is agreed upon. This crutch is that there is such a thing as "Maximal greatness" in regard to God.
The counter to the ontological argument is typically given by using an example of an object or food, such as a pizza. If we can conceptualise a "maximally great" pizza in our minds, then the argument would follow that one actually exists in the real world. As we know, this would be ridiculous, but that would only be the case because a "maximally great" pizza is indefinable due to its subjective nature.
One person may believe that a thick crust is a key component of a "maximally great" pizza, whereas another person may believe that a thin crust is superior. We can assert from this that the argument may only work if the subject can be attributed the status of being "maximally great" in an objective manner—or at least in a manner that can be argued for objectively, as anything that is subjective would result in its existence being logically impossible.
Unlike a pizza, the values of God do have intrinsic maximums. A pizza could always be larger or have an extra topping to make it that slight bit greater, whereas God’s qualities cannot be exceeded in the same way. There are inherent maximums for certain qualities, including moral goodness, power, and knowledge, which make up the idea of a maximally great being. For instance, it is conceptually impossible to know more than what is required for perfect knowledge, which is the knowledge of everything that is true. As we can conclude that it is at least logically possible for God to exist and that the maximum value attributes are definable, it must therefore be true that a God does exist, by extension of the ontological argument.