If God did not exist then would people even have come up with a concept of God?

Right, but many unbelievers exist in just such an imaginary realm, so the question we are asking is, "Is that imaginary realm internally consistent?" Obviously, the pipe-dream paradigm of naturalism is impossible according to the objective rules of reality, but here we're showing that it's impossible even by the rules of the modified (un)reality that it itself embraces because there is a huge gaping inconsistancy.

Michael