Full Idea
It seems apriori coherent that there could be a necessarily existing god, and that there could be no such god - but they can't both be true. Other examples include unprovable mathematical necessities
Gist of Idea
Contradictory claims about a necessary god both seem apriori coherent
Source
Laura Schroeter (Two-Dimensional Semantics [2010], 2.3.4)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.29