I'm going to begin by answering the first question as it might relate to the alleged healing powers of acupuncture. Then, I'll draw those threads together and show how the same considerations actually apply to God, too.
The idea behind acupuncture is that the pathophysiology of the disorder being treated includes an unhealthy bodily flow of Qi ('Chi') - said to be a type of living or vital energy. The placement of subcutaneous needles in specific locations is supposed to restore the normal flow of Qi, helping to heal the disorder in a biologically active way.
Qi is an ancient and intuitively appealing concept. There must be some important difference between living organisms and dead ones (or inanimate objects), after all, so why wouldn't it be a mysterious quantity and why not call it a type of energy since, in other circumstances, energy is invisible except for the things it makes happen? But we now know better. Vitalism has been thoroughly and completely discredited by science. Life is driven by the usual types of energy that are all well described - the same kinds of energy that drive all chemical reactions, only in the case of life, those reactions maintain homeostasis for at least enough time for a given species to reproduce. No additional or special type of energy is required to explain life, and no reputable or serious biologist thinks otherwise, anymore.
Nevertheless, millions of people still believe in Qi and acupuncture. "Science just hasn't discovered a way to detect or measure Qi, yet," they tell us. But that is a big, smelly, red herring.
It actually doesn't matter whether we can detect or measure Qi itself, for we are told that Qi and its manipulation has effects in this world that are measurable. We are told that acupuncture has the measurable effect of healing people. Accordingly, we can conduct randomized controlled trials where people are randomly assigned to real or sham acupuncture treatments (where the needles don't actually penetrate the skin and are placed at random locations by non-acupuncture practitioners). If acupuncture works, we can make a prediction: the people getting real acupuncture treatments should improve more quickly and/or more thoroughly than the people getting sham acupuncture treatments. It doesn't matter one iota that we can't measure Qi or it's flow patterns directly. All that matters is that we can predict and measure the alleged effects of Qi here and now, in the natural world.
Well, studies of this kind have been done over and over and I'm afraid that it doesn't look good for acupuncture. The outcomes in the 2 groups are largely indistinguishable. One clearly shouldn't think of acupuncture as doing anything biologically active beyond the power of suggestion. While this doesn't disprove the existence of Qi, it certainly proves that acupuncture as a way of beneficially manipulating Qi is useless. Maybe there are other ways of doing so, but until those are discovered, the idea of Qi adds nothing to our understanding of illness and health, and there's absolutely no reason to believe that it does exist.
It's possible that Qi has nothing to do with anything in the natural world. Perhaps it's purely supernatural. But, if it is, then it's of no consequence here, and should be of no concern to anybody; the whole idea is without meaning. If it does have consequences in the natural world, then those consequences should be measurable or detectable.
And so it is with all supernatural claims, including the existence of the Christian God. Maybe we can't directly detect the Christian God, but we can reason from God's alleged qualities to predictions about the way the world ought to be, and then look for evidence of whether the world is as we'd expect, or not. We would expect a universe where the Christian God exists and is omnipotent, morally perfect, perfectly loving, desirous of a personal relationship with us, etc. to look quite different from a world where no such God exists. For example, if the Christian God exists, we wouldn't expect any gratuitous natural suffering, yet we see a world that appears to be overflowing with it. We wouldn't expect there to be billions of non-believers clustered within borders explained by natural and haphazard factors like politics and conquest. If evidence gathered in the world is better explained by the non-existence of God, then the existence of God should seem much less likely to us. At the very least, it should cause us to become very skeptical of the alleged qualities that those failed predictions are based upon. There are very many predictions made by Christian theism that can be tested here on Earth, and I'm afraid that, like acupuncture, the situation doesn't look good at all.
Perhaps the Christian God doesn't exist, but a different God who lives entirely in a theoretical supernatural world does. Perhaps we know nothing about the qualities and capabilities of such a God and he never interferes in this world in any way. This pretty much describes what deists believe. This type of God really is beyond investigation by reason and evidence, but what a useless belief! A universe where this God exists is no different in any way than one where such a God doesn't exist. It might as well not exist at all.
This reminds me of the parable of the invisible gardener, by John Wisdom:
"Two people return to their long neglected garden and find, among the weeds, that a few of the old plants are surprisingly vigorous. One says to the other, 'It must be that a gardener has been coming and doing something about these weeds.' The other disagrees and an argument ensues. They pitch their tents and set a watch. No gardener is ever seen. The believer wonders if there is an invisible gardener, so they patrol with bloodhounds but the bloodhounds never give a cry. Yet the believer remains unconvinced, and insists that the gardener is invisible, has no scent and gives no sound. The skeptic doesn't agree, and asks how a so-called invisible, intangible, elusive gardener differs from an imaginary gardener, or even no gardener at all."I'm afraid that existential questions seem to always and only boil down to reason and evidence. When there is reason and evidence, a conversation can be had about their merits and meanings. When reason and evidence are unavailable in either principle or practice, we have a meaningless claim that terminates the conversation.
Believing despite insufficient reason and evidence - believing on faith - is propped up as being incredibly valuable, but why? What's so great about faith? It seems to me that in every domain of human discourse other than religion, believing on faith is rightly frowned upon. Would you cross a street on faith without looking to see if a bus is coming? Look at what believing in the existence of God on faith gets us: thousands of Gods and traditions most of which are mutually exclusive, and balkanized doxastic communities with a horrible and ongoing history of intolerance, discrimination, and slaughter in the name of those beliefs. This is supposed to be the Zenith of human understanding and the path to the most important 'truths' in the universe? What could the word 'truth' possibly mean in that sentence without reason and evidence?
There may well be plenty more in Heaven and Earth than is dreamt of by reason and evidence, but without them, I'm afraid it's all just "Words, words, words." (Shakespeare - Hamlet).