My previous post - “Show me the Money” - implies that I think direct evidence is required to remove all doubt on the existence of something that has been claimed. To address the other channel by which a claim can be agreed upon, let me discuss “rational” argument using the example of the Argument from First Cause . Of course, people "debating” the existence of God - or any other supernatural contrivance - will generally not have a rational argument in the sense that I use it here, but it is worth treating it as if they do.
The arguments for the existence of God often share some common characteristics:
1) They rely on ignorance. This is not meant to demean anyone - we are all ignorant about the vast number of topics there are to have knowledge of, let alone those things that we don't know can be known. The First Cause argument relies on ignorance of the cause of the universe, and can be dumbed down to “we can’t explain existence, therefore my preferred explanation applies”. The fact that we can’t explain existence does not trouble me personally, so it could be that the ability to remain untroubled by this is one of the personality traits that make non-belief so easy for me. Personal trivia aside, assume that no one knows what caused the universe. What would motivate someone to think that our absence of knowledge can only lead us to a conclusion that “God” is responsible? Not only does the assumption that something other than what can be observed must be the cause **complicate** the explanation instead of simplify it, it then is made worse by slapping on all sorts of other characteristics that magically parallel the proponent’s personal beliefs and (most often) their chosen religion. That same approach also ignores all other explanations - supernatural and natural. More than that, we really have an argument from ignorance, a false choice, a bare (unsupported) assertion, some circularity and a violation of Occam’s Razor - all in one argument.
2) Having just mentioned the False Choice fallacy, let’s discuss it next. Assuming (as we are) that no one knows what caused the universe, the possible explanation should not be limited by a personal preference. For instance, why not Zeus? Why not David Berkowitz’s neighbor’s dog Sam? Why not a cosmic joke? Why not a quantum fluctuation? The cold reality is that the universe is as it is, and none of our personal preferences or biases are relevant in the grand scheme of things. We may someday understand where our observable universe came from. We may never understand it. That we might like it to be a certain way - for instance with an omnipotent father figure in charge - will not and can not have any bearing on reality. So the potential explanations for the question “what caused the universe” are logically open to anything that is not logically impossible. It is not logically impossible that a God exists, but it is extremely more improbable than a purely natural explanation. No matter how improbable the universe may truly be, the existence of a system beyond the universe, in which a God can exist in order to create and be in charge, is enormously more unlikely. All purely natural explanations are thus more likely.
3) The Bare Assertion Fallacy is an “arbitrary dogmatic statement which the speaker expects the listener to accept as valid”. No evidence or rational argument is offered, that’s just the way it is because I said so.
Obviously, you can reject this without further discussion. They’re just expecting you to accept them as an authority. Unless you have reason to, this is a non-starter.
4) Circular Reasoning is a direct outcome of Bare Assertion. In fact, the two are joined at the hip. You can’t assume in a premise, what you’re trying to arrive at as a conclusion.
5) Since I already smuggled Occam's Razor into the discussion of False Choice, let me give it a slightly better treatment. Using the example that Dr. William Lane Craig often uses - that the complexity of the universe is too great to be explained by natural means - let’s examine how God might be a more likely (probable) conclusion. As we know, “...among competing hypotheses, the one that makes the fewest assumptions should be selected”. This is just a matter of probability. If you have a one-in-six chance of rolling a six using a single die, than adding a second die lowers the probability that a single throw will produce a six. There is one way to roll a six using one die - or 16.67% probability. Two dice produce 5 ways out of 36 in which to roll a six - or 13.89%. This is over-simplistic, but makes the point. To really put an exclamation point on it, you can argue that God is a die that is more complicated than the universe, thus requiring more faces, thus lowering the odds of rolling that six even further.
Summary: Arguments about God don't prove anything. More generally, arguments between people, and organized debates don’t prove anything - they just attempt to persuade the participants or audience to change their opinions. After all is said and done, we’re still stuck with the fact that the universe does not care about our opinions.