We would be a perception type existence produced by our brains whether God exists or not is irrelevant, to the fact, we are an idea type existence.
If you mean the human brain is capable of abstract thought, ideas that have no direct counterpart in nature, like "heaviness" and "Captain Marvel" and "2" and "unicorn" and "justice" then yes, concepts are a major part of how our brains have evolved to think. Language works by labeling concepts; that's why translation between eg English and Chinese is possible (though if you've ever tried to translate poetry, you'll know it involves substitutions of concepts that are more or less similar, but not the same).
But that doesn't mean things that are purely conceptual or imaginary are real, in the sense of existing, independently of the concept of them.
All the evidence says the only place such concepts exist is in individual human brains.
Idea as in produced program/thought in the mind, who we are is not a physical thing.
Our knowledge of how the brain works advances constantly; and we've begun to understand how it works, though indeed we still have a long way to go, But nothing we've found suggests that anything 'spiritual' or 'supernatural' or 'immaterial' is involved at any point,
Things are not real just because we can imagine them, have the concept of them. Things are real because they exist in nature, that is, in the world external to the self.