This is an excerpt from a debate I've been having on Keith Devens' web log. The following was written in response to the claim that all atheists are actually agnostics.
Here are two of my core arguments. I will divide definitions of god into two kinds. The first is unverifiable and meaningless, and the second is naturalistic and unworthy of worship.
God Type I
How do we determine whether a proposition has any meaning? We can create propositions by stringing nouns, verbs and adjectives together, but following the grammatical rules of the English language is not sufficient. Pick some words at random, and you may get a proposition that is grammatically correct, but nonsensical, e.g., "humidity cogitates on libertarianism". Similarly, you cannot have a meaningful proposition created out of symbols that are undefined, e.g., "x = (5 p + 4 q) * 7".
So the question is, where does semantic meaning come from? I argue that when we learn language (even our native language), we are creating theories about what words and propositions mean. The first time you ever saw a guy say "what's shaking?" to his friend, and his friend reply "not much," you created a theory about the meaning of the expression "what's shaking?" Since you saw nothing physically shaking, you theorized that the expression meant "how are you feeling?" or "what is going on in your life?" You tested this theory by observing other people use the expression, or by testing it out yourself.
Our entire system of language is built out of many such theories, each one confirmed by empirical evidence.
So, we create theories about the meaning of every proposition, and these theories can only be confirmed or falsified by experience. If you deliberately construct a proposition whose theory of meaning can never be confirmed or falsified, then the proposition itself has no meaning.
If you claim X is true and that nothing we ever experience will ever confirm or refute X is true, then I can make the exact same claim for X is false. If you claim God is good and that nothing we ever observe can convince us otherwise, then I can equally well claim that God is evil and that nothing we ever observe will convince you otherwise. This is because you are saying that the meaning of God is good has nothing to do with anything we ever experience. If you believe such things, you have fallen into a trap where everything you see is interpreted as evidence for your proposition, and nothing can be seen as evidence against it. This is a fallacy. An observation, O, cannot be construed as evidence for a proposition, P, unless not observing O is evidence against P.
Now what I have said above applies to the traditional Christian God which is defined as being omnipotent, omniscient, eternal, perfectly good, etc. All these superlatives are stated to be unverifiable even in principle. So, all propositions about this god are nonsensical. We may intuitively think we know what they mean, but we are actually confused. Intuition alone is not an adequate guide to the meaning of language.
God Type II
The argument about semantic meaning does not apply when you are talking about a super-powerful alien being with technology so far above ours as to look like magic. For, in principle, we could attain technology comparable with the alien, and do experiments that expose his parlour tricks. However, neither you nor I would consider such an alien to be god, per se.
A second aspect of atheism is rejection of worship of pure power. Even if an alien created our universe, that would not justify our worshipping that alien. Might does not make right. Suppose that the devil was actually the one god/alien who created the universe. Would it be ethical to do evil because he says so? Would evil then become "right"? I don't think so. In other words, what an alien says is right isn't necessarily so. I don't think that it is right to hold slaves or to stone people to death like the Bible says we must. Even if I believed that the Bible wasn't just made up by humans, I still wouldn't agree with what it has to say. To follow a path arbitrarily specified by the almighty, I would have to sacrifice my conscience and become a collaborator.
In summary, I have made two points here. First, if your propositions about god are not falsifiable (not even in principle), then they are nonsensical. Second, if your propositions about god are falsifiable, then the god you're talking about is an alien subject to the laws of nature (even if those laws are different outside of our universe). In the latter case, you have to not only say why you think that alien exists (the burden of proof is yours not mine), but also why that alien is a god, i.e., why that alien deserves worship.