I apologize for practicing inferior epistemic hygiene. Thank you for indirectly bringing this to my attention. I knew that the quote was commonly attributed to Nietzsche, but I had never seen the original source. It would seem to be a rephrasing of this quote from The Antichrist:

The fact that faith, under certain circumstances, may work for blessedness, but that this blessedness produced by an idée fixe by no means makes the idea itself true, and the fact that faith actually moves no mountains, but instead raises them up where there were none before: all this is made sufficiently clear by a walk through a lunatic asylum.