The problem of the question is the definition of the term "god". Some liberal religious people believe that "god" is not a being, but more a feeling they have during rituals or social interactions, even music.
If you define "god" as such a manifestation of a feeling, you could prove the existence of "god" by measuring the brain activity of such a liberal religious person.
Such a definition of the term "god" is used by liberal people who accept scientific facts but want to hold on to their tradition. I think the definition of "god" is one of the major problems for modern, liberal theologians.
Because the term is so badly defined, it is hard to discuss it properly. (That may be the reason, why so many of our atheist arguments focus on extreme definitions of the term.)
I define "god" as a supernatural being or process of some sort. I am totally convinced that nothing "supernatural" exists, whatever it may be. Supernatural means, it is beyond natural processes and out of the reach of science. Rational argumants can only disprove predictions, that are made by religious world views. They can show that natural (!) processes lead to certain phenomena. This can disprove a "supernatural" theory indirectly. A direct disprove of a supernatural idea is literally impossible, because it is out of the reach of any rational method that could prove or disprove.
Therfore we can't be sure 100%.
I don't agree.
Something out of reach from any rational analysis, would be something that we couldn't see, hear, taste, smell, or detect in any way...
In other words, we would have absolutely no reason to believe in it. (The teapot that orbits saturn falls into this category).
But god does not fit in this category... every single religious scripture affirms that god interacted with our "natural" world, so he is not out of reach of any rationality. Simply seeing him, or one of his miracles, would be a rational method of proving him.
However, it is a fact that divinity, god, miracles and related terms, have all been invented at a time where we had no idea of how the natural world worked.
Now we know better, and these ancient terms simply have no more reason to exist.
Science will never discover a supernature* so I can be certain that any god ever imagined by anyone in the short history of our species does not exist. The supernatural does not exist. I am certain that nothing happens beyond nature.
Because I can see no proof either for, or against, I can't be sure there is no god. There IS a problem with that, because if there was a creator, even if all he did was start the Big Bang, then who created the creator? The concept of eternal existence is hard to wrap my mind around, which is why I have to say I just don't know.
On the other hand, I AM positive that there is no god who sticks his finger in our affairs and meddles with them. There is too much horror, both natural and human-made in this world for me to believe that god cares about the sparrow who falls (a Christian concept). It is obvious to me that prayer is useless, although enough coincidences happen that I can accept that people believe their prayers have been answered. I also think that belief is a security blanket for people who desperately hope that nothing bad will happen to THEM.
I also understand that our human religious heritage comes from our primitive ancestors who had no scientific way to view the world, and were afraid of things that go bump in the night. I understand their desire to explain how the world came to be as best as they knew how, and their desire to see bad events as punishment from their gods, because then, if they were good enough, maybe god would treat them better. Too bad they could never be good enough.
The only sad thing is how those primitive beliefs have persisted into the modern world. And that otherwise intelligent people truly DO believe them. But it's clear that they have never managed to banish that primordial emotional fear response, and so, need to manage it by sucking their thumbs and holding on to that blanket for dear life.
100% positive if you round it to the nearest integer.
Am I ever so arrogant as to say I know anything that hasn't been proven to be false but very unlikely, is 100%...nope. I'm basing my opinion on the information at hand, and that information overwhelmingly tells me that there is no god. If there were evidence discovered to the contrary, I would have to change my mind, but until then, I'm not loosing sleep over this question. I don't deal in absolutes, and quite frankly, I'm not sure that's a good way to be. I accept as fact, that, that has been proven via Science and Math, and I'm open to anything it may prove in the future. I'm not willing to tell anyone I'm a 100% positive without that type of proof, only what I believe based on the facts at hand. Truth is relative I guess. Not always rational for sure, but rationality is something I wish more people counted on. To me it's highly more likely there is no god based on the information I've seen, and that is what I'm comfortable believing.
I don't think it's a bad thing to be 100% sure, just as long as you admit your mistake if proven wrong.
I have an idea of the world that seems quite logical to me, and there's no room for divinity, ghosts, or green furry marshmallow men that walk around at night hiding my car keys.
So I am 100% sure they do not exist. (but this doesn't stop me from being open-minded)
If someone were to show me proof of one's existence, or a compelling reason to believe in one's existence. Then I'd be happy to turn around and modify my world-view so that nocturnal green car-key-hiding marshmallow men have a place in it...
though it would be quite awkward... me having no car keys...