You may be operating on a deeper level than me here, but it seems like that Fulton Sheen quote just claims that being mistaken or ignorant about whether or not you are God makes you "a lunatic". Whether you use that terminology or just replace it with "incorrect about stuff", it still seems distinct from being "a liar".

(I think we're in agreement that being mistaken about things is not incompatible with being a "great teacher", as I say in another comment here.)