I think your best argument against the existence of God is just the Occam's Razor type thing where you say it isn't necessary to complicate things by introducing a God into the equation because you can explain perfectly well without one. I think when you actually get down into it, it's a simpler explanation to have a God in the equation, but I can see why not everyone agrees.
Balto, could you expound on this point?
If my model includes only the Universe, and your model includes the Universe + God, then how can your model be simpler? It necessarily includes all of the complexity that is in my model (i.e., the Universe) and then adds another entity to the equation.
The response I've heard from theists is that my model is inadequate because it fails to explain: (1) creation
ex nihilo; or (2) why the Universe's physical constants are fine-tuned for life. I don't find either of these arguments persuasive.
The first argument faults my model for having a first link in its causal chain. But that's not a valid criticism, because the theistic model suffers from the same shortcoming. It adds a cause that precedes the Universe, but there is still a first link in the chain. The only change is that now the first link has the name "God."
Of course, people have lots of rationales for why it's valid for God to be the first link. They say that he is eternal, that he has no cause, that he originated outside of time, that we simply can't understand his origin, and so on. But all of these theories can just as easily be applied to the Universe itself. (i.e., the Universe is eternal, the Universe has no cause, the Universe originated outside of time, we simply can't understand the Universe's origin) Thus, the model gains no additional explanatory power from the addition of God.
The second argument is dispatched by modal realism and/or the Everett Interpretation of quantum mechanics. If numerous (potentially infinite) Universes exist, then it makes perfect sense that life will only arise in those Universes where all the necessary preconditions have been satisfied. Our existence can be understood as the result of a completely random process, without any need for supernatural volition.
A theist might counter that my Multiverse model requires belief in entities that we cannot directly observe. Of course, the same is true of God, so the real question is which explanation to prefer. And there is at least indirect support for my model, based on evidence like the double-slit experiment.
From the perspective of information theory, the Multiverse explanation is infinitely less complex than God. God has all sorts of arbitrary attributes that would need to be specified, like his moral preferences and his plan for the world. In contrast, the entire Multiverse can be derived from an algorithm that says "all possible things exist."
In a sense, this provides an even better answer for the creation
ex nihilo question. The two simplest algorithms for specifying existence are "nothing exists" and "everything exists." The former obviously is not operative, given the fact that somethings exist. So Occam's razor dictates that it's logical to choose the simplest remaining choice: Everything exists.
So in a sense, I do believe in God. I think that somewhere in the Multiverse, there are entities whose composition, powers, and motivations may resemble those of God (or at least, demigods). But I don't believe the design of the entire information structure can logically be traced back to a volitional entity. And I see no evidence of these pseudo-Gods at work in our corner of spacetime.