> Hopefully my tone isn't misinterpreted
No worries :) I love these kinds of conversations.
I'll give a personal anecdote before I start slingin' scientific/philosophical language: My de-conversion started when, as a teenager, I asked myself if I only believed in Christianity (instead of, say, Islam or Buddhism or Hinduism) because I was raised in the church--and that if I'd been raised in some other religion, I would instead believe that. (I'll use this example to better frame the null hypothesis question later, but it's useful to keep in mind as I explain.)
> What experiments have you performed to prove the "null hypothesis"?
This is a minor misunderstanding of what a "null hypothesis" is and how we interact with it. A null hypothesis is basically the assumed-default of the world, based on the fewest assumptions. Any theory you may be investigating may be an alternative hypothesis, but the way you go about collecting data is less about proving an alternative and more about disproving the null.
To be maximally pedantic, science is about constructing models that most accurately model the real world. If the data contradicts a model, then it clearly isn't reliable, and we make adjustments to the model--so we would never "prove" any hypothesis, null or otherwise, we would just find that the data more and more supports a specific hypothesis (over some others).
This philosophical block of text from Wikipedia explains why we frame the null in this way pretty well:
A negative claim is the opposite of an affirmative or positive claim.
It asserts the non-existence or exclusion of something.
The difference with a positive claim is that it takes only a single
example to demonstrate such a positive assertion ("there is a chair
in this room," requires pointing to a single chair), while the inability
to give examples demonstrates that the speaker has not yet found or
noticed examples rather than demonstrates that no examples exist
[from 1]
So when applied to our god claim, it looks something like this: I see people making saying things like
(a) god has had influence on our past (in dictating his laws to people, performing miracles, the resurrection etc)
(b) god has influence in the present (he can communicate with people, either directly into their mind or by influencing the world around us; enabling certain outcomes to occur, like getting a specific job or having a good surgery outcome)
While (a) is a little hard to test 'cuz I don't have a time machine and the historical record is opaque at best, (b) is a series of positive claims that can be observed and tested against the null of god not existing or not having any influence over our reality.
Were I to invert the null, and assume that there is a god, it's possible I could construct my experiments poorly: suppose I pray that I'll get a raise at my job, and the next week my manager tells me I'm getting a raise. Is this evidence that god exists and is influencing my life? Perhaps my boss was just motivated out of my work performance over the past few months. If I'm assuming that god exists, I could easily count this as evidence without that being the case.
Suppose the opposite happens, and I don't get my raise: What does this have to say about our god question? It might be that my raise "doesn't fit god's plan", and he had his own reasons not to grant my request--the typical explanation I hear from believers. It could also be simply that I did not meet my boss's expectations for deserving a raise and it was unreasonable for me to expect one, godly influence or no.
If you loop this over a lifetime of experiences, it's very probable that you'll run into a series of events where god seems to be at work, i.e. you pray for things and they happen, or you have some fortunate luck occur, or whatnot, that you're happy to attribute as evidence to god existing--even if there's perfectly mundane explanations for each of these occurrences. You could be wrong that there is a god, yet merrily believing such because of this poor logic chain. Broadly this is "confirmation bias," [2] or "only counting the hits." You see this with things like when you buy a new car: suddenly you start seeing that car everywhere. In reality, nothing has changed in terms of the percentage-distribution of cars in your local area: you're just now more predisposed to notice the kind of car you have than you were before, so it "feels" like there are now more of them.
You may ask (and apologies if I'm straw-manning), "What if God's actions are simply indistinguishable from the mundane chance?" In science we would say that said effect would be "statistically insignificant"--if you cannot reasonable separate the effect of some thing from random chance, then it's unreasonable to conclude that the "some thing" has influence on what you're observing.
You see this in medical trials with what's called "the placebo effect" [3], where doctors will run a study where some group of people get actual (new) medicine and some group get sugar pills, and yet another group will get nothing; there are many, many cases where the sugar-pill group (the "placebo" group) display effects _different_ than the "control" group who got nothing. If the results of the group who got real medicine are similar in magnitude to those who got the sugar pills, then it'd be unreasonable to conclude that the medicine is having a real effect, since that effect can also be demonstrated by not-real-medicine.
To your questions:
> Doesn't that assumption introduce some amount of skepticism and bias into the measurement?
You're absolutely correct :) Some atheists also use the 'skeptic' label, as it more widely describes their approach to life beyond just their religious beliefs. Bias I would question--would a similar bias not be introduced by assuming there were a god? (I'd argue I've demonstrated quite a significant bias would be introduced.)
If you remember my anecdote from the beginning, I had the question of if I only believed Christianity because I was raised in it, vs these other religions. One way to frame the experiment would be to assume Christianity was correct, and begin collecting evidence: but this would leave me open to the "confirmation bias" of attributing events to a god who may not actually exist. Defaulting to the null hypothesis of there not being any god, and requiring myself to be convinced otherwise, allows me to be much more sure that *were* such an existence to be demonstrated, that it would indeed be true with regard to reality.
> How do you respond to those who claim they've experimented on their faith and found sufficient evidence to assert their belief?
Generally I ask them to walk me through their beliefs and their experiments, in the vein of Street Epistemology. I highly, highly, highly recommend watching a few of Anthony Magnabosco's videos [4] on the subject. He does an amazing job of taking a calm, reasoned, conversational approach and demonstrates many of the techniques and tools I've cited and talked about, with a variety of people on a variety of claims--though typically claims about God. In the interest of full disclosure, Anthony is an atheist, but these conversations are much more of a Socratic dialogue exploring beliefs than they are a debate.
Returning to your question, I have not yet had a conversation with a believer whose evidence I found convincing, typically due to flaws in their reasoning akin to the above. A few months ago I even read several books one such believer recommended to me ("Mere Christianity," "The Problem of Pain," and "Should We Fire God?") on the basis that those books were part of what helped build their faith, and I only came out the other side even more disillusioned about their ability to rationally engage with the subject.
> I've found that starting this particular hypothesis under the premise that there is a God who has crafted a plan that requires the use of your agency to regain His spirit...
This is a bit of a self-fulfilling prophecy, with regard to confirmation bias: If I begin looking for signs that there is the Christian god, and I allow myself to count what would otherwise be mere coincidences (or possibly very likely outcomes that I mistakenly though unlikely), I would merrily begin finding this evidence: not because it's good evidence, but because my premise is flawed. Suppose alternatively there is a god, but it's the god of Islam, or of Judaism? A lot of that evidence could support the Christian premise, but still not lead me to the correct conclusion if I'm starting from the Christian default.
I currently see no reason to believe that there is any god with such a plan requiring me to suspend my reasoning--and I would charge that any god that required me to suspend my reasoning in order to be led to believing in him (especially if that belief is demanded, lest I be punished eternally) is a god of poor moral standing.
> What if the proof requires some amount of willful credulity?
Then it's a remarkably ineffective proof :)
[1]: https://en.wikipedia.org/wiki/Burden_of_proof_(philosophy)#P...
[2]: https://en.wikipedia.org/wiki/Confirmation_bias
[3]: https://en.wikipedia.org/wiki/Placebo
[4]: https://www.youtube.com/watch?v=T2EgJjQRsYE&list=PLh10RgQgGu...