Prior to the invention of telescopes, many gazed into the night sky and concluded that celestial bodies were gods. Saturn was so important, a day of the week was named after this god (hence, Saturday). Here's a close-up of Saturn:
Mars, the god of war, second only to Jupiter. We sent our robots to Mars to investigate.
Nope, just another planet, this one actually fairly similar to earth.
According to Romans at one time, this was the god of sky and thunder and king of the gods until Christianity became dominant. Let's see Jupiter up close:
Makes sense to worship what turns out to be the provider of all the energy needed for all life on this planet, and probably why we observe Sunday as an important day of the week. Turns out it's nothing special, it's just another sun in the galaxy. It is special to us, however, so feel free to worship it. Just don't expect any special favours (unless you consider skin cancer a favour).
I could go on, but so far, I've demonstrated that gods were observable phenomena that, while not understood at the time, were not things outside the human concept of understanding. You should also understand that I have seen all of these gods through my own telescope, so I am not taking anyone's word for granted (yes, I also have a filter so I can also observe Sol - our sun.)
Now, the next argument usually follows that these are ancient Roman gods, and don't count, because nobody worships those gods today. So, where does the concept of a modern Christian god come from? Ask a believer today, and you'd think you're asking them to nail jello to the wall. However, the scriptures do not lie, and the effects of religion on modern day culture has left its mark.
In this case, I think Aron Ra made the best case for what the modern Christian god really is:
Yes, air. When I sneeze and you say, "Bless you," you are citing an age-old tradition in ensuring that, as my air spirit escapes in the form of a sneeze, demon spirits will not enter my body due to your blessing. When we pour a glass of hard alcohol, we see vapours rising from the top, which is why they became known as "Spirits."
The current god that is observed by western culture is an air god, but we have come to learn what air is made of and why we need to breathe it. This collection of molecules is clearly not god.
The agnostic will insist to push the concept of god beyond our understanding, but this is because god has traditionally been beyond our understanding. Before the invention of telescopes, we did not know what Mars, Jupiter, or Saturn were, but they were things within our understanding because they were observable phenomenon. Even air, although invisible, is observable in its effects and through powerful microscopes.
To postulate that a god could exist outside of our understanding is in contradiction to what we can currently observe about known gods. Every single god has existed through some observable and knowable phenomenon, and therefore can eventually be understood. Whenever somebody postulates that something could exist outside of our observable and knowable knowledge, that thing cannot be a god by any reasonable understanding of what constitutes "God." While the agnostic may postulate that things beyond our ability to observe and understand could be god, they fail to understand that, if something cannot be observed and understood, it is not a god in the sense that the intellectual have come to understand what gods really are. In fact, it becomes something completely different. A lie, perhaps; or maybe just a delusion.
I'm certain we'll discover many more things that were previously unseen to us. Science has done that; it has shown us germs, ultraviolet and infrared light, radio waves, and X-rays, just to name a few. None of these turned out to be gods either, but if they were observable without instruments, I'm certain they would have been considered gods and given their own names as well, because that is what gods have always been: Those observable phenomenon which we did not understand. This is why I am absolutely certain that gods do not exist.