You can't know that something doesn't exist, unless its impossible, like a three-sided square that's simultaneously red and blue all over!
I can see a tree outside my window. I know that it's external, because other people can see it and because I can't make it go away. Of course, we can be mistaken: the whistling sound in my right ear is not external, although it sounds like it, because no-one else can hear it and it started after inner-ear disease. And not everything is accessible to everyone. For me, the leaves on trees are mostly brown and the "go" traffic light is grey, but I take other people's word for it that they are some mysterious thing called green.
Think about it, though. If there were no books and people's claims, how would you conclude or even be agnostic about something that "may" exist? That's like concluding that the dishes I just washed in my kitchen is siting next to me in my bed room on the desk. Is it there? No. I know this. I can't "see" it. There is no proof that says it isn't there. Out of my logic (1. washing the dishes in the kitchen 2. Puting the dishes in the dish rack 3. Walking out bare handed) why would I assume or even be agnostic about dishes on my desk that in my brain computes doesn't exist?
People can write endless books about the dishes on my desk. They can share their experiences about washing dishes in the kitchen and poof! they see the dishes on their desk. Others can sense their are objects next to them regardless the logic of them walking out of the kitchen with
nothing in their hands. My brain doesn't work that way. If two and two is four, I'm not going to be agnostic and think "maybe it is five because we can't prove it isn't even though we have clear evidence in history and human thinking and just logic that it is four."
Knowledge of gods non-existence has to do with understanding human history, chronology, the human psyche, and just a whole lot of things that make up why and how we believe what we do from tradition (regardless if it's Pagan, Christian, Buddhist), to I don't know any number of things.
What would give me a reason to think a god does exist?
What we experience is internal. That's basic psychology. What we perceive and how we interpret our experiences are not just how we react to it externally (say get angry and hit your boss) but internally (say feel your boss is plotting against you that's why you think you got fired).
It is completely natural and part of the human psyche. There is nothing wrong with that.
I can see a tree outside my window. I know that it's external, because other people can see it and because I can't make it go away.
This means objective rather than external. You can see a tree regardless if other people see it or not. However, because other people see it, it's not "just your imagination." For some reason, no one takes people's word for it unless X amount of people seen it too.
Why would we not externalize our strong personal feelings and beliefs? It's a natural reaction. I experienced (internally) the thought my grandmothers (or spirits, one of the two or all) push me back from getting hit by a car. That was my first thought and I know (belief confirmed by fact) that it is true. The experience is external because I literally was pushed back. However, without my externalizing it by whatever means as we do in all our psychological thinking, then it would just be thoughts. However, our thoughts sometimes do bring on actions.
In this case, it was an external stimili (the car) that brought on the external action (the push) that lead to an automatic conclusion that (grandma/spirits) protected me. It's alright that it is both origin from our mind and outside our mind.
Many people believe god is external. He comes into the soul of a person
and then they get internal feelings of love etc. Their feelings are a confirmation of the existence and presence of god.
For me, it's the other way around. Everything originates from the mind. How we perceive things and interpret things. So my external experiences (tree being green to, I don't know, my experience with the sacraments of Christ) brought on internal experiences. Those internal experiences (rather than external) confirmed what I experienced is true.
People always call people crazy for things like believing in god or signs of depression they are not educated in knowing the symptoms of. The mind is a powerful tool. If we think our internal experiences (say god) are external, than it's defeating the point of what spirituality is. It's not
just eating food and drinking water. The lifestyle is a part of your being.
That's why I feel it's idolism etc to see the Bible, Quran, Lotus Sutra, etc as sacred objects because is starts internally. The source is internal. Call it soul, spirit, or psyche, doesn't matter.