There is nothing to show God doesn't exists, either. We're basically at a cross-roads of simply choosing what we will believe, or lack belief in.
That is not true, not even a little. If we take your God, the Christian one, and we look at his holy book and then compare that to what we know is true now we'll see that a lot of the stuff that's in there isn't true.
God did not made man out of clay a couple of thousand years ago, man evolved over millions of years. We know that for as close to a fact as can be without getting trapped in the 100% certainty of a dogmatic mind.
We know that Noah didn't build an ark that held two of every animals on earth. We know that there wasn't a global flood 5000 years ago.
There are of course many more examples of things that are complete make-believe that's in this book, supposedly, containing the word of your God. If a lot of it simply isn't true we have a reason to doubt everything.
The same goes for every other religion, current or not. And with all the religions not worshiping the same god or gods, we can assume that most of them are wrong. But you can't really stop there, if every other religion is wrong, then chances are yours is too.
NOW, if you skip the ridiculous notion of a personal theistic god that created mankind out of clay and then threw him out of Paradise for eating an apple and instead argue the existence of a deistic god that created the universe and the laws that it obeys, that's a different story. Or a god that is everything. Or call the laws of the universe God.
How can you know for sure? What you're doing is stating a universal negative.
I hope you understand that.
I don't deal in absolutes, that's for religious folks. I take the evidence we have and I compare it to the dogma of religions. If they don't match, then that religion gets discarded.
If the Abrahamic religions claim man did not evolve but was created by God out of clay, then I discard that as nonsense.
If they say the Earth was flooded 5000 years ago and we can't find evidence that the Earth was flooded, then I discard it as nonsense.
Add together enough nonsense and I have no reason to believe any of the stories. Prove that the stories are
true and you have a case. The Testaments are not a reliable source and does not hold up as evidence for the stories in them. Just because they are written do not make them true.