If we don’t have any direct experience of God, and the idea GOD isn’t innate, then in some way, we have invented it. But even if we have, that doesn’t mean that God doesn’t exist, i.e. that the concept doesn’t refer to anything. It is possible that we need the concept GOD, which we have come up with, to explain what we do experience or to explain the existence of the world, or something like that; and that these explanations are right.
In science, we often invent concepts, which we think refer to things – like the concepts GENE or ELECTRON – which we haven’t experienced. Both these concepts were invented in order to explain what scientists saw. No one had seen a gene or properly identified its structure when it was created – as a hypothesis – to explain what happens in heredity. And no one can see electrons (with their eyes at least) at all! Just because we invented the concepts doesn’t mean that they don’t apply to the real world. If genes and electrons exist, then we were right to invent the concepts, i.e. the explanations we gave which used these concepts, and how we define GENE and ELECTRON are the right explanations and definitions; this is how the world is. (You might argue that, even if we couldn’t see genes or electrons when we came up with the ideas, we can see them now. But can this reply be made for all scientific concepts? What about LEPTON or, in a different way, ECOSYSTEM?)
What is important here is that we invented the concepts because, in some way, explanations using them are true. So the reason for the concept is to describe the world truly. Historically, people explained lightning and storms, wars and famine as God’s actions. Perhaps the need to explain the world is where the idea of God comes from. And one question that perhaps suggests the concept GOD most universally among human cultures is ‘where did the world come from?’. Every culture has a creation story, and in very many, God plays a part.