It's all about the deep questions.
Theism: Ontological Arguments
These arguments are most likely to be completely foreign to the lay person, and if they were presented with them, they would most likely dismiss them as a simple word game. Personally, I do not know of one case where a non-theist was persuaded to become a theist by them. In my opinion, they are among the weakest of all theistic arguments. Regardless, in the interests of completeness I will present two such arguments, Descartes and Plantinga’s. I will be very brief.
Descartes Ontological Argument
1. God is the most perfect (‘the greatest’) being conceivable.
2. It is more perfect (‘greater’) to exist than not to exist.
3. Therefore, God must exist.
Criticism 1: Kant’s critique
The mere concept of God does not entail its existence. One cannot define something into existence, which is exactly what this a priori reasoning amounts to. Existence is not a property like “all-powerful” is. For example, the existent invisible pink unicorn does not exist. Therefore, the second premise is simply false, since existing is not a perfection.
Criticism 2: Hume’s critique
I will simply quote. “I shall begin with observing, that there is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by arguments a priori. Nothing is demonstrable, unless the contrary implies a contradiction. Nothing, that is distinctly conceivable, implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no Being, whose existence is demonstrable. I propose this argument as entirely decisive, and am willing to rest the whole controversy upon it.”
Criticism 2: Cue the parodies!
1. A perfect island is the greatest island conceivable.
2. It is more perfect to exist than to not exist
3. Therefore a perfect island exists.
Of course, island can be substituted with anything to proved the existence of a perfect anything! Therefore, perfect baseball pitchers, perfect societies, perfect husbands, perfect wives, etc. all exist! In this manner a huge number of parodies can be constructed. Since all of them follow the exact same logic, and they are all false, then the original must be false.
Plantinga’s Ontological Argument
Say that an entity possesses “maximal excellence” if and only if it is omnipotent, omnscient, and morally perfect. Say, further, that an entity possesses “maximal greatness” if and only if it possesses maximal excellence in every possible world—that is, if and only if it is necessarily existent and necessarily maximally excellent.
1. There is a possible world in which there is an entity which possesses maximal greatness.
2. (Hence) There is an entity which possesses maximal greatness.
Criticism 1: Begs the question
Within the logical frame of the argument, assuming premise 1 is tantamount to assuming God exists. It also falls prey to the objections of Hume and Kant.
Criticism 2: another parody
1. There is a possible world in which there is no entity which possesses maximal greatness.
2. (Hence) There is no possible world in which there is an entity which possesses maximal greatness.
In the first argument assuming premise 1 led to theism. The non theist can just run the argument in reverse. In this argument, assuming premise 1 leads to atheism. The argument is a non-starter.