Consider the following principle:
WEAK PMC: Possibly, every concrete object (and aggregate of such) that has an originating or sustaining efficient cause has an originating or sustaining material cause, respectively.
In simple terms, WEAK PMC says that it is possible that all made things are made from other things. A bit more carefully, it says that there is at least one possible world in which all concrete individuals and stuffs that are made are made from other concrete individuals or stuffs.
Now my own view is that a much stronger version of PMC is true -- viz., that it holds of metaphysical necessity. But there are strong grounds for thinking PMC holds in at least the actual world: it's intuitive, it satisfies standard accounts of justified conceivability-possibility inferences, it has no uncontroversial exceptions, and it's encoded in the well-confirmed conservation laws of physics. A fortiori, then, there is sufficient evidence to warrant the claim that there is at least one possible world W in which such a principle is non-vacuously true. But if so, then in W, some things or stuffs are made, and all made things or stuffs are made from other things or stuff. And if so, then no things or stuffs in W that are made are made ex nihilo, in which case no god or gods made them ex nihilo in W. But on classical theism, for any world that contains concrete things or stuffs distinct from God, at least some of those things or stuffs were made ex nihilo. It follows that the god of classical theism doesn't exist in W. But if so, then by (i) the fact that classical theism entails that God is a metaphysically necessary being, and (ii) Axiom S5 of S5 modal logic, it follows that such a God doesn't exist in any world, and therefore, a fortiori, such a God doesn’t exist in the actual world.
