A flea hears from other fleas that there such things as mountains. Being a rational and inquisitive flea, it decides to set out and see for himself if it is so.
A fleas segmented eyes are made so that it can only focus on things a few inches away. Everything beyond that is a blur, so it cannot see any mountains. Fleas have acute senses of smell and taste, but since mountains have no smell or taste the flea can't detect any mountains that way either.
Fleas have moderate, but limited intellectual properties, so our flea is unable to create a tool to detect mountains or devise a system to deduce the presence of them either. The way fleas are made, he can start at sea level and walk straight up the side of a mountain and never know it.
Because he can neither detect, nor deduce the presence of mountains, the flea decides that there is no such thing, and the fleas who say there are mountains must be either deluded or fibbing.
This particular flea though, lives in the wool on a goat living in the Alps. He has, in fact, lived his entire life on a mountain, even though he has no way of knowing it.
Human beings have set out to discover God in the same way as this flea. Some of them, because neither their senses, nor their tools, nor their reasoning can detect God, have decided that there must be no God, and anyone who believes there is must be either deluded or fibbing.
Fleas cannot detect mountains, but that's not an accurate test of whether there are mountains or not. Likewise, humans cannot detect God, but that's not an accurate test of whether God exists or not.
A flea may be correct if he says he doesn't believe in mountains because he cannot detect them, but he goes beyond his bounds if he says that there is no such thing as mountains.
Likewise, a man may be telling the truth if he says he doesn't believe in God because he cannot detect God, but he goes beyond his bounds if he says there is no God, because he has no way of knowing whether there is or not. Hubris leads us to believe the only things to exist are those our meager senses can detect or our limited intellect can deduce.
Just like the flea who lives on the goat who lives on the mountain, I believe we would be amazed at the remarkable things that do exist but are beyond our ability to detect them.
Image: Mountain Goat Statue Near Corviglia - St. Moritz, Switzerland