This is a real classic.
Imagine that you're a frog in a well.
To you, the world is the sky which you can peer up to, the walls
around you, and the water you dwell in.
By no prior knowledge, or whatsoever, you can make a sound
assumption that the world is made up of the sky, the walls, and the
water - it fits perfectly fine because there are no compelling
evidence of any other things that makes up the world.
We are the frog in the well when it comes to viewing the universe
and beyond. You could readily disprove the existence of God just by
trusting the compelling evidence you have, but i would hold back my
statement for there is still much for humanity to learn about the
bigger world out there. 
