An instructive example is the famous failed theory of the luminiferous aether, the hypothesized medium and absolute reference frame (for the uniform constant speed c) of light. Wikipedia has thorough coverage, but the quick overview is that theoretical predictions weren't confirmed by experiments and competing theories with greater generality arose, i.e. relativity and wave-particle duality. Could the luminiferous aether exist? Maybe, but if it does then nowadays nobody has any pragmatic rationale to use it in mental or physical tasks. They can think and act on the basis that it isn't there. (Ethernet doesn't count.)
A god(s) "theory" is subject to similar pragmatic evaluation. All that's needed is to figure out and run the corresponding tests of reality. Unfortunately, this approach is typically not applicable by the religious. How come?
- Foremost is a taboo against questioning religious propositions. Of course, the emphasis on the taboo can vary, and novices of the religion may ask questions as a normal component of instruction. Befuddlement is permitted. "Attacks" and/or suggested alternatives to the core essentials, especially by members who should "know better", is prone to responses ranging from insistent re-education to ejection. Not all groups are heavy-handed and hierarchical but most will take steps to forcefully prevent the spread of "corruptive lies". Admittedly, this feature is not peculiar to religion. Groups united by common beliefs, i.e. pretty much every lasting group there is, probably will remove naysayers for the sake of survival, akin to an immune system. Otherwise, the group wouldn't in fact be united by common beliefs!
- After a god test, the attendant religious measurement and interpretation is equivocal. Given that posited gods have freedom to make decisions, each test must be unsure. Either the test passes or fails, and either the god decided to show up for the test or not. And due to godly free will, the god and the test are statistically independent, more or less. That is, the correlation is zero between the test's actual outcome and the god's existence. For instance, as long as the god can possibly choose to not participate, all failed tests might mean nothing more than that. Thereby the god tests are inconclusive. According to the popular definitions, free will cannot be predictable because a decision known beforehand isn't "free". The more that an observer knows about the decision-maker, the more predictable the decision appears, and the less "free will" could be involved. A human who's known to be compassionate tends to predictably perform compassionate actions when the chance arises. Is this free will at work? What if this human, unknown to the observer, underwent tragic personality-changing brain trauma in the recent past (there have been well-publicized cases)? The ignorant observer could watch the human decide to act contrary to compassion and then exclaim "I see again that I can't predict what someone will do! That's free will at work!" What's perceived as free will could be a lack of knowledge. The nice occasionally are mean on their worst days, but who knows whether today qualifies? In like fashion, when a god test fails, and so the interpretation must be the god's exercise of free will, the "scope" of free will is customarily enlarged through greater and greater presumptions of a lack of knowledge about the god's decisions: "It doesn't make sense for my god to make that decision; my understanding of my god's qualities must be much more incomplete than I estimated". In the face of many failing tests for a god, the believer claims less and less to be attuned to the god's utterly mysterious and unlimited free will.
- Mystery is far from a problem in religion. It's more likely to be honored, for without mystery there is no opportunity for faith. The religious scoff at the proposal of god tests, which serve no purpose for the faithful. Their god(s) simply exist, regardless of the experience or reasoning of lowly humans. Religion is concerned with great celestial realities, hence present human reality need not be entirely consistent with it. Temptation is an enemy, yet doubt is the most fatal. Letting doubt roam free potentially endangers the great panacea, faith. Intellectual discontinuities shall be overpowered by emotional flurries of trust and hope. Take a deep breath and let your troublesome mental conflicts gooooooooooo...
I don't wish to be seen as negligent for not addressing the primordial god test: isn't a universe an adequate clue that a god must be around? My answer is no. The problem with a god-creator theory is its startling incompleteness. It's really closer to being a sketchy conjecture. There's a solitary data point, which is the universe. That's not much to work with in order to distinguish between theories. What would be different if a god started the universe, as compared to, um, something else? To what can I compare the purported act of creation?
Moreover, this idea is very vague about the god. "This picture was painted." "By whom?" "The Painter, naturally." The creator-god is a riddle wrapped in a mystery inside an enigma. Were there many creator-gods working together or at cross-purposes? Is the creator-god in the same form as at the time of universe creation or did it empty itself into its creation? Is it what humans call intelligent or a clumsy oaf committing accidents? Is it needy and/or accomplishing a goal? Is creating an effortless hobby? These are valid albeit mischievous questions. Remember, the metaphorical Painter could have been a hyperactive dog whose paws had firmly-attached rectangular paint sponges.
I suppose that I could be persuaded to hypothesize that 1) the universe started through an inscrutable process, whose placeholder label is "creation", and 2) the process was initiated and overseen by an inscrutable thing, whose placeholder label is "god". But after accepting the hypothesis merely for the sake of debate, it amounts to bupkis. It certainly doesn't follow that "universe-creation god" is all-powerful or all-knowing or immortal or a close personal friend of a proud species that scrambled to the undisputed top of the food chain on planet Earth orbiting around star Sol somewhere in the Milky Way galaxy (sincere apologies for the terribly sloppy address).