This essay attempts to provide a plausibly sound argument for theological incompatibilism, where (i) theological incompatibilism is the thesis that if theological determinism is true, then for any created agent S, any time t, and any true proposition p, it is not up to S at t whether p and (ii) theological determinism is the thesis that God’s willing what God wills necessitates and explains every other contingent fact. While the argument offered here is similar to other arguments for the same thesis, it is unique insofar as it invokes the technical notions of thorough ontological priority and metaphysically generative relations. In introducing and defining these technical notions, this essay draws on recent philosophical discussions of explanation, grounding, and ontological priority.