I wonder if fish believe in airplanes.
Airplanes are simply not part of a fish's universe. Most fish live in the water all of the time, and most airplanes don't go into the water. There are fish who jump out of the water, and there are fish who may have been caught on a fishhook or by a predator and escaped back into the water. But in general, fishes are in the water, and airplanes are in the air.
Now, certain fish might occasionally have a certain kind of encounter with airplanes. For example, on a sunny day a fish who happened to be looking up might, by chance, see on the surface of the water the shadow of an airplane which happened to be flying by. Or, a fish who lives in a place where a seaplane lands might have a more direct experience with an airplane, maybe get to touch its pontoons. An airplane might even enter completely into a fish's realm of experience if it crashes into the ocean. Of course, even a theoretical Very Intelligent Fish (VIF) would not likely be able to explain these experiences very well. A VIF would have to be in the right place at the right time to experience Airplane, and although he might be able to predict some occurrences of Airplane (say, if it was a seaplane that landed at a certain place and time every day, or if the ocean was near an airport with regular takeoffs and landings) he would never be able to make Airplane happen, or truly explain what it was. He would only be able to experience it for himself and report what he experienced to other VIF. There would be varying theories as to what Airplane is, what it means, and how best to experience the mysteries that Airplane brings, but most of what Airplane is would remain a mystery.
Some fish would never in their lifetimes experience Airplane. Maybe they stay deep in the depths of the ocean where sunlight never goes. Or maybe they just don't happen to live in a lagoon where a biplane lands, or near that noisy airport where they could hear the rumbles and see the plus-shaped shadows. I imagine, though, that VIF that believe in Airplane and wanted to experience it might tend to congregate in places where Airplane might be experienced with some regularity.
Or maybe not... after all, we're just talking about worm-sucking fish here.
But fish, even VIF, would be making a mistake to say that airplanes don't exist at all just because they don't exist under the ocean. Airplanes inhabit a realm of existence that fish simply cannot go without help. It would be a very misguided VIF that would claim that because he hadn't ever experienced an airplane (and hadn't even tried) that he had authoritative proof that Airplane is a myth.
Airplane is real.
...And anyone who has had a genuine experience with God can tell you, even though God does not originate in or physically inhabit our realm of existence, God is still real. Occasionally God penetrates into our cosmos, and those who believe in Him sometimes gather (we call it church) to meet with others who believe in Him and to seek out those experiences, but we can't make God do things at our commands, any more than a fish can charter a flight to Disney World.
Maybe there really is not a God. Even after all of the times in my lifetime that I've had experiences with what I believe to be a divine being, and even though I believe that being to be the God described in the Bible, I can't offer you physical proof of his existence. I don't have his autograph, or the jersey he wore when he played basketball in Junior High. He's not from here; He doesn't live here.
Just because airplanes aren't submarines doesn't mean there are no airplanes. And just because God doesn't physically live in the house next door to me doesn't mean there's no God.