Not necessarily. God has effects on the natural world. Otherwise we would never know about him. In principle that provides an opportunity to verify that he exists.
One attempt was intelligent design. It didn't claim to know what God is, but did claim to show some kind of intelligent designer outside nature. The attempt failed, because there are reasonable ways that evolution could have formed the examples cited. But some kind of proof like that might possibly be convincing.
Also, I'm not sure just what "natural" means in this context. God is certainly not an object in our universe. But suppose that cosmology learns about something outside our universe, and that it finds something that looks like God there. I don't see that this would necessarily contradict the Biblical picture of God. I'm not sure whether this would make God "natural," and why I would care if it does. One conventional picture of God is that he necessarily exists, and that he created the universe. If something exists and is able to create universes, it seems to me that at least in principle we could find out about it.