How do you know God exists or doesn't exist?
You don't, not for certain. That's why some people say that agnosticism is the only valid option. But you can look at what the arguments are and make decisions based on probabilities. And at the start of that process you need to differentiate between good and bad arguments, strong and weak evidence, and to set aside rhetoric, which is really just the dressing up of your case to make it look as good as possible. As an ex-preacher I think I know something about rhetoric! (I once borrowed some sermon notes from a senior colleague of mine. In some places there was underlining in red and I asked him what that meant. He said "Oh, that's 'Argument weak. Shout.'")
Evidence is strong when it can be independently evaluated and repeated, and when it's available to inspection. It's weak when that isn't so. We can all test the boiling point of water, some other things need specialised equipment and training, but even they are in principle open to inspection. If, on the other hand you tell me that you've seen someone spontaneously burst into flames in the street, you'd need some very convincing back-up indeed as it is our general experience that this doesn't happen.
If I ask our religious companions for the evidence for their beliefs there is none that is open to inspection in the normal way. Religion is a matter of conviction: indeed it makes a virtue of the absence of evidence by saying that belief without evidence (i.e. faith) is in some way and for some reason virtuous.
There was a time when belief in some unseen personal force or other must have been very attractive. Consider for a moment one of our very early ancestors asleep in the jungle at night and suddenly there is a sound - he doesn't know what. He can say "it's just a sound" or he can say "that sound is caused by something or someone who may be malevolent". In the first case he ignores it, in the second he takes some steps to ward it off. These are, of course, as effective as the guy who kills crocodiles in the Sahara. If you point out that there are no crocs in the Sahara he has the answer "you see how good I am?" However, be that as it may... if the ancestor who ignores the noise is wrong just once, and it is caused by someone or something malevolent, then he's dead. But the other guy may be wrong 99 times out of a hundred, but if he's right once then he survives. So you can see how this tendency we all have to attribute agency (something is done or caused by some agent or other) has a strong survival value.
It works the same way in an agricultural community. The crops fail so we perform rituals and sacrifices to the gods. The crops return, so the performances have pleased the gods and we can make a note and do them again. If the crops stay failed, the gods must still be angry, so we had better try harder, or try a different ritual, or a more costly sacrifice. And if it works, then it's case 1 all over again. If it fails, we go in for another iteration. And of course, eventually, the crops return. But the category mistake is to think that a correlation (we did this and the crops returned) is a causation (our doing this made the crops return). We human beings are very inclined to make that type of category mistake. What you notice in this iteration is the wonderful way the priests of the god concerned have for justifying themselves - it's infallible. Eventually the ritual "works". Except that it is a category mistake.
I know that absence of evidence is not supposed to be evidence of absence, but if you're told there is an elephant in your fridge, you open it and look. In the absence of evidence (no visible elephant, no footprints in the butter) you make the only possible deduction that there is no elephant.
In the same way, I don't know there is no god, but I can find no strong evidence for a god. Such evidence as is provided by religious people is always weak and usually incredible. Sometimes they say, as Jung did, "I don't need evidence, I know". But anyone can say that, a madman as well as a religious visionary, and we have no way to tell which is speaking. So I take the view that it makes most sense to act as if there is no god and I am responsible for my actions to myself and my fellow human beings alone. If someone can bring the required strong evidence, then of course I have to be open to change my mind.
