The existence of a god is something that can't be disproven. You can always find gaps in knowledge and explain the gap by saying a god / multiple gods did that. As gaps narrow with more knowledge, you can always just say that the holy books were just a metaphor in this one case, but the rest of it is literally true.

It gets even more complicated when you run into people who refuse to believe in any science, or anything outside their own personal experience.

Personally, I believe the Earth is a sphere. I've been to Australia, Europe, Africa, Asia and North America. The time the flights took and the routes the in-flight maps showed make sense for a spherical earth. So did the scenes visible out the windows, and the day/night cycle. The mere existence of time zones and seasons strongly suggests the Earth is a rotating sphere tilted slightly off vertical. But, it could be that I'm living in a Truman Show world, where everything is a lie designed to make me believe something that isn't true. I haven't personally done all the math, all the experiments, etc. to prove the Earth is a sphere. And, if this were a Truman Show world, the producers of the show could mess with my experiments anyhow.

For someone who doesn't want to believe, there's really nothing you can do to make them believe. The world really relies on trust and believing Occam's Razor.