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. Not, of course, to a priest: for his instincts prompt him to the lie that sickness is not sickness and lunatic asylums not lunatic asylums. Christianity finds sickness necessary, just as the Greek spirit had need of a superabundance of health — the actual ulterior purpose of the whole system of salvation of the church is to make people ill. And the church itself — doesn’t it set up a Catholic lunatic asylum as the ultimate ideal? — The whole earth as a madhouse? — The sort of religious man that the church wants is a typical décadent; the moment at which a religious crisis dominates a people is always marked by epidemics of nervous disorder; the “inner world” of the religious man is so much like the “inner world” of the overstrung and exhausted that it is difficult to distinguish between them; the “highest” states of mind, held up before mankind by Christianity as of supreme worth, are actually epileptoid in form — the church has granted the name of holy only to lunatics or to gigantic frauds in majorem dei honorem. … Once I ventured to designate the whole Christian system of training in penance and salvation (now best studied in England) as a method of producing a folie circulaire upon a soil already prepared for it, which is to say, soil thoroughly unhealthy.
Note: This quote is often summarized and attributed to Nietzsche on the web as: “A casual stroll through the lunatic asylum shows that faith does not prove anything.”