According to the Copenhagen interpretation a quantum system remains in superposition until it is observed. If God was omnipotent he would be all knowing, implying that he observes all. However since the cat's state remains in superposition we can infer that it has not been observed, and therefore God is has no knowledge of the cat's state. That contradicts the initial assumption that God is omnipotent, but if God is not Omnipotent then he is not God at all, and therefore God does not exist.