what if water was not H20., ,

 The theist just needs to concede that, for all she knows, God may not exist. 

A helpful analogy comes by way of reflecting on the idea that water is H2O. 

While there are no metaphysically possible worlds where water is not H20, for all one knows, water is not H20. 

Hence, there are epistemically possible worlds where water is not H20 (Chalmers 60-62)