Gödel had an unpublished ontological "proof" [1] that's fairly simple to understand. It hinges on two things: 1.) accepting a logical system in which possibly necessary truths are necessary; 2.) accepting his argument that it's possible for god to exist.

But I guess you said non-crazy.

[1] https://en.wikipedia.org/wiki/G%C3%B6dels_ontological_proof