Begs the question - I would posit that the minimum assumption for any form of 'spirituality' is body/mind duality, and your proposed 'better' definition of insanity presupposes the result that there is no axiomatic, logical system that can result in body/mind duality being either true, or undecidable.

However, so long as it is even undecidable, then a person that uses it as an axiom for further thought is no more 'insane' than someone that explores the logical consequences of parallel lines crossing.

Now, Religion posits not only body-mind duality, but a number of other assumptions, and those other assumptions are generally quite amenable to debunking. But I suspect dualism itself qualifies as undecidable, which would place it outside the pale of propositions which one can both maintain a cohesive logical structure and explicitly deny.