There are various argumentative strategies for advancing the claim that God does not exist. One such strategy is this. First, one notes that God is meant to have a certain divine attribute (such as omniscience). One then argues that having the relevant attribute is impossible. One concludes that God doesn't exist. For instance, Dennis Whitcomb's recent paper, ‘Grounding and omniscience’, proceeds in exactly this way. As Whitcomb says, ‘I'm going to argue that omniscience is impossible and that therefore there is no God.’ However, if having a given property is impossible, the greatest possible being need not have that property. Accordingly, the argumentative strategy in question is doomed to failure. The upshot of this article is a quite general one concerning how arguments against the existence of God in fact must proceed.