Insanity manifests itself in believing that something is true against the available evidence without any new supporting evidence.
A man claims to be Napoleon. Authorities show him the clear evidence that he is not; he offers no credible evidence for his assertion, and he continues to believe he is Napoleon. The man is insane (is manifesting his insanity).
Religious belief is belief against the evidence without any new supporting evidence. Religious people manifest their insanity. They need treatment. If only psychiatrists were nothing better than snake oil salesmen and witch doctors.