In the debate of whether or not God exists, which is often used to ridicule those that do believe in God as simple idiots who have no brains, the premise if often thrown out the there is no 'evidence.' So I'd be curious to hear what exactly would qualify as evidence?
You would presumably need some sort of internal "evidence" to justify a "belief." But justifying a personal belief in God is not the same as making a truth claim upon the beliefs of others, so the "evidence" required is very different.
In some branches of philosophy a validity claim must be grounded in a "true, justified belief," around which a great thicket of qualifiers, definitions, and controversies has arisen. In scientific matters or legal cases, "evidence" conforms to various formal specifications, the possibility of falsification or replication, and ultimately some formalized, yet conditional consensus.
But these are cases of "public" evidence, "experiential" confirmation, aiming at consensus. No such evidence can be offered for the existence of God. The existence of sacred texts, the testimony of believers, or witness of "miracles," are not instances that can be readily falsified or confirmed by repetition and collective experience. A "miracle" as Hume notes, is precisely that event which cannot be repeated, demonstrated to others, and thus confirmed as a "law of nature."
So if you ask what evidence "you" need to believe in God, the answer is simply anything that convinces you. An inner certainty, an answered prayer, a near death experience, an ontological argument, personal testimony of a loved one, a sense of dread, a startlingly apt line of sacred text, Pascal's wager, a warm feeling in the chest area, a lack of psychological alternatives.
Anything could qualify as "evidence" for "your" belief in God. But "evidence" that compels agreement from others in the manner of mathematical proofs or scientific and legal evidence is notoriously lacking in the case of divinity. And is probably not applicable even in theory. I would add, however, that John Locke, the father of modern empiricism, considered the four separate eye-witness accounts of the Gospel as good as legal evidence gets. Further historical-linguistic evidence has superseded that happy opinion.
Postscript. Since I am having trouble with "Comments" function, I'll respond here to a comment below. The "historical-linguistic evidence" is simply about the Gospels, which were not eye-witnesses, were not independent (three being traceable to "Source Q"), have proven inconsistent, and were selected from many contemporary apocrypha. None of which was known to Locke. What gives these research findings more weight than "someone's happy opinion," is simply the conventions and standards of "public" or "scientific" evidence discussed above. This may or may not have anything to do with those personal beliefs we may label "opinions" or with the "evidential" constitution of such beliefs. My point is that Locke's argument would can no longer qualify as valid "historical evidence," as it might have in his day. You seem to be equating "evidence" with "truth." The working conventions of public or scientific "evidence" are highly constrained, limited, and functional. One can argue about "absolute truth" or "God," making your case. But such arguments are simply misplaced in the laboratory or law courts with their widely adopted standards of public "evidence."
mega, we all have faith in something or 'nother, whether we want to admit it or not.
for me, "evidence" of the existence of God lies simply in the existence of the Universe and of reality and our existence in this Universe as beings of consciousness, sentience, and sapience. now "evidence" is not proof and, in this argument never will be.
this evidence is this very Universe that shows multiple signs of having been created with purpose. this is called "teleological" reasoning or the "teleological argument". it is not conclusive evidence that will force, from the imposition of logic, an honest doubter to adopt the belief. but it can be sufficient, for someone who chooses to believe in God, to have confidence that their belief is reasonable. but there will always be counter-arguments that the doubters will use to justify their doubt. (or, if i were to be less generous, that the deniers would use to justify their denial.)
you'll never win an closed-minded doubter over with any argument regarding God. whether the argument is weak and trite (and there are plenty of those) or reasonable (i won't say "strong", i don't believe there is an undeniably strong argument either for the existence of God or against the existence of God.)
i might point out that the Strong Anthropic Principle (SAP) requires quite a leap of faith itself and they will respond with the Weak Anthropic Principle (WAP) which is essentially a tautology or truism.
the WAP has to be true, because it's a tautology. and, because it's a tautology, it doesn't say much. it's like saying 5=5. a kinda "empty truth" (and i know i am semantically misusing that term, but i don't care; i am trying to make a point.) then they will say, "Well, the WAP provides a perfectly natural explanation for Fine-tuning by showing that the human perception of Fine-tuning is simply an example of selection bias.
then i will say, "For selection bias to suffice in removing any notion of the remarkability of Fine-tuning of the Universe, that requires a large collection of universes from which to select a life-friendly universe." then they will say "Yeah! Q.E.D!"
they may even point to different mathematical models of the multiverse, some of which are logically consistent and may even be true. but we will never know. because, for the same reason i will never create a God-measuring-device or a falsifiable physical experiment that will show, one way or 'nother, that God exists (or not), neither will they ever create a similar falsifiable experiment demonstrating the existence of any universes outside of the observable universe.
so it's fine to believe in the Multiverse. it might even be true (and even if true, that would only push the argument of infinite regress back a level; i would argue that God created the Multiverse). but i don't need the existence of the multiverse to have an understanding of the existence of me.
however, what if this forever unmeasureable Multiverse doesn't exist? what if the Universe we live in is the only one? that appeared some 13.8 billion years ago (perhaps as a hell-of-a quantum fluctuation)? well, for the 26 or so fundamental physical constants to come out just right so that sufficient carbon was cooked up but for the same process to not fit so well in cooking up oxygen that all of the carbon would have been converted to oxygen, for there to be the other elemental diversity so that small rocky planets such as Earth could exist for life to emerge and beings like us to evolve and wonder how we got here, for all of this to be set to happen 13.8 billion years ago, causes this conscious, sentient, and sapient being to wonder if all of this creativity must have come about from purpose. as opposed to purposeless and "undirected processes".
they can believe in their unproveable explanation and i can believe in mine. and, when you boil it down, both require "the substance of things hoped for, the evidence of things not seen", what some unknown author 2000 years ago defined as "faith". and that is all the proof you can hope to get out of it.
So I'd be curious to hear what exactly would qualify as evidence?
First you have to understand what evidence means: Empirical observations that support one explanation but disqualify other explanations.
Evidence for belief would be some feature of the world around us (something in nature, something about the way human beings are made, something about the laws of physics, etc...) that can only be explained by the existence of God, or for which the most reasonable explanation is God.
The cosmological argument and the teleological argument are basically attempts at providing evidence based arguments for God's existence (as opposed to the ontological argument which is based on pure logic).
Those who disbelieve view such arguments as unconvincing, stating that modern cosmology and the theory of evolution provide better explanations for the way the world is than the cosmological and teleological arguments.
A key difference for atheists is that science has predictive power whereas belief in God doesn't: Both science and God work as after the fact explanations for the way things are, but only science allows people to predict future events. This predictive power is seen as evidence that science is more accurate than belief in God.
People saying that you need evidence for belief have a weird understanding of the word belief. As do people that say their belief was 'true', because truth is connected to knowledge and what you know, you don't need to believe.
See "About meaning, knowing and believing" in Kant's Critique of Pure Reason, B 531-38.
People like Richard Dawkins et al. are asking for scientific evidence, see here. That contradicts the notion of belief in God, but they do not care. They question the need for religious belief in modern times in general.
The main point against them could be that belief (=faith) provides certainty beyond doubt while knowledge cannot. Empirical certainty has to be looked at as doubtful since Hume. The problem is that their certainty that they are right comes from their belief in the superiority of the scientific method, which is in fact unscientific, because science should be sceptical empiricism, as Hume was the first to state, not scientific realism.
Inspired from @robert bristow-johnson I have an alternative reading now:
The problem that occurs between atheists and theists may also lie in the use of language and slight equivocations.
Dawkins et. al. seem to say that they would need a (scientific) evidence for the existance of God because a belief would explicitly have to be a belief about facts in the world. And the sort of certainty about facts in the world needed for this belief is only provided by science (sceptical empiricism).
On the other side, a theist may be able to dodge this attack by saying the very notion of a belief in God would be that faith is all you need to be certain. A coherence with inner sentiments and his personal being is all "evidence" there can ever be. And perhaps even more, that in the moment you are looking for evidence in the world, you lost your faith, you are doubtful and not reachable for God's voice anymore.
In this sense, they are talking about different spheres of reality and are not contradicting each other at all. And perhaps Dawkins would have to be seen as reductionalistic, because faith in fact has its reality in human experience.
On the other side, every theist saying that God would be in the sense that he is a fact in the world could be seen as counterfactual and dogmatic, because talking about "the world" means talking about the sphere of experience that is shared by all of us ("objective" nature), not only the faithful ones. It would be a residuum of ancient times where more or less all people had to be (or pretend to be) faithful, so that there was not difference between the reality of nature and the reality of God. This is what the New Atheist movement is attacking (rightfully).
But denying the existance and reality of faith is overdoing it. It is scienctific realism: only what science can "proof", is an ontological entity. And this is reductionalistic and falls through the old body-mind problem. It is falling back behind phenomenology and pragmatism, into the 19th century (of the history of philosophy).
Can't make a comment on the original question yet - but this isn't a duplicate of the other question mentioned in the comments - that one focuses strictly on how a religious text/book might be able to prove that a god wrote it. This is much more general to any evidence
Evidence for God or god(s), other than purely philosophical constructs, comes primarily in the form of legal/historical proofs. While there is science involved in aspects of proving the historical nature of religious artifacts, writings, etc., primarily it is based on testimony and circumstantial evidence cases.
First class - historical proof of religious artifacts / writings / people. For example, was Jesus a real person, and what facts from the Bible can be reasonably concluded to be historically accurate from the account. Same question for the writings and people involved in the founding of any religion. Was Mohammed real? What is the likelihood we have the original Quran or Bible intact, does the historical evidence (archeology, etc.) support the text, or is there doubt in it's historicity? How much is myth, how much is real?
Second class - historical eyewitness testimony. Mostly related to historical documents (i.e. if we believe, for example, that the Gospel of Luke is more or less historically accurate - whatever we believe of the miracles therein, then it is an account of a man who had direct eyewitnesses as sources). The question here really goes to the veracity of the witnesses, and what you believe about whether they are telling the truth or not, and if the recorded accounts are accurate in part or whole. What were their motives, etc. Similar to whether a juror believes a witness in court or not (whether they were intentionally lying, misleading, omitting, telling the truth, etc.). Any case where there is a report, but the people involved cannot be cross examined.
Third class - personal accounts from people we have direct access to, today. If someone experienced a healing, for example, can we talk to them, access their story? Can we follow their healer around and document new cases? This also relates to people that have a religious experience of any kind. If I know of people who have "become saved" and have changed their life, and I talk to them about it, their testimony is evidence to me - not just what they say, but also how their actions/life has changed can be evidence
Fourth class - personal experiences of the supernatural or "presence of God". This class will not be a large evidence to other people (though it might have minor impact - see Third Class above), it will, however, have the largest impact on you, the one experiencing it. This could also be true of witnessing a miracle, but not being the direct actor or receiver of it.
Basically, historical/legal arguments require that a case be built up, and each person will have to determine separately what their level of proof is.
For more information on the general theory (i.e. legal/historical proof of religion), plus a large amount of historical / legal evidence specifically related to Christianity / the Bible, see "Evidence that Demands a Verdict" by Josh McDowell.
[This answer is currently incomplete.]
I will concentrate on the Christian God, since this is the one that I know best. However, similar comments apply to many other religions and their deities. Hereafter, by "God," I mean the Christian God.
To create a case.
To create a scientific case that God exists, you would have to perform five basic steps.
Step 0. Identify a body of phenomena that science cannot currently adequately explain.
Step 1. Find out whether these phenomena really do occur, either by reading the existing literature or by going out into the field and conducting the experiments yourself. Basically, make sure you're not just deluding yourself. Be sure to maintain a healthy disregard for anecdotal garbage (ah, sorry I mean anecdotal evidence) at all times.
Step 2. Specify a Christianity-inspired scientific theory that purports to explain the aforementioned phenomena. Lets call this theory T.
N.B. The specification of T has to be done very clearly, and you have to list your basic assumptions. For example, these might include the assumption that God exists, that he/she has certain specific motivations, and certain specific abilities, etc. No evidence is required at this stage. What is required is a great deal of clarity. You have to be so very clear, that at no point do I ever have to consult the Christian bible or learn any Christian doctrine in order to understand what your theory T is claiming or predicting.
Step 3. Use theory T to convincingly explain the aforementioned phenomena. This explanation should be so clear and insightful, that it produces new predictions about the relevant phenomena. It should also be so clear and insightful, that it suggests the existence of new phenomena that nobody ever thought of.
Step 4. Go out into the real world and perform experiments until you've ascertain whether or not theory T's new predictions/phenomena actually match up with reality.
To attack such a case.
Even if you successfully get this far, and all the evidence so far accumulated seems in favor of theory T, there's still some avenues of attack that critics may take.
Avenue 0. Show that theory T makes contradictory predictions e.g. it both predicts that we will observe P, and also that we will not observe P.
Avenue 1. Find an alternative explanation. For example, perhaps a Hinduism-based (or Buddhism based, or biology-based, or psychology-based...) explanation is able to make equally-good, equally-accurate predictions. Perhaps it does even better.
Avenue 2. Show that God is non-essential to the success of theory T. For example, perhaps we can use Occam's razor in order to cut away a new theory T' that also makes all the correct predictions, but without assuming that God exists. If so, then God wasn't integral to the success of the original theory, and hence the correctness of the predictions of the original theory can't be taken seriously as evidence in favor of God's existence.
How it plays out in practice.
Step 0. Lets try to explain how it is that people are able to heal other people by prayer. In short, the phenomenon we're interrogating is: "prayer works." But, we're discounting prayer for one's self due to the obvious issues arising from the placebo effect.
Step 1. We read the literature, taking care to only trust reputable studies. So in particular, we only trust randomized controlled double blind trials, and only when the experimenter's are neither especially religious nor especially anti-religious and don't appear to have a conflict of interests. We observe that the occasional study finds that praying for someone else does cause an increase in the probability of that person achieving positive health outcomes, but not by very much; the effect is tiny. The occasional study finds that prayer has a negative effect on the probability of achieving positive health outcomes; but again, the effect is tiny. Most studies fail to find any effect whatsoever. Hence, it appears that there is little to no overall ability of prayer to cause more positive (or negative) health outcomes, and the occasional studies to the contrary are easily explained as statistical outliers. Perhaps overall, the literature seems to be detecting a slight tendency of prayer to help more than hinder, but this is easily explained by the massive problem of publication bias. You go on to read some meta-analyses that confirm your suspicions that prayer has little to no effect on actual health outcomes, and these analyses also cite publication bias as the likely cause of the tiny effects that they have found.
At this point, we're starting to feel discouraged, our eyes are blurring and its hard to function. You burn your toast in the morning, and its too late to try again. Even asking the local waitress for the drink you want ends up being a prolonged struggle, and you keep mis-ordering and having to correct your order. Then when you go to pay her, you drop your change everywhere and the baby nearby nearly eats it, and you just feel really bad about the whole thing. In practice, this is how you realize that its time to give up on this particular plan of attack. Perhaps its time to create a whole new case for the existence of God based on explaining an altogether different set of phenomena. In any event, you're done!
But. Just for the sake of argument. Lets pretend that contrary to everything I've just said, we actually find that prayer does work; more precisely, that the effect is so large, that neither publication bias nor poorly controlled experiments nor scientific corruption can explain it.
Its a miracle!
But, as it turns out, our celebration was incredibly premature.
To be continued...
First, the word "believe" as it is used in religion, precludes evidence. A true believer believes without evidence. If they had evidence, that wouldn't actually help them believing at all.
As a non-believer, god or "a god" is supposed to be an almighty being. I can imagine things happening that could be caused by humans. I can imagine things happening that could be caused by a civilisation much superior to that of humans. Any such thing wouldn't be prove for the existance of god.
There would be a limit, where some effect couldn't possibly be produced by anything in the known universe. Which doesn't mean the cause would have to be a god, but we might as well assume it is, since having a force that is more powerful than anything possible, but less than god, is quite hard to imagine.