Well,
JOE
was narrating about faith, which is about believing without evidence. Also,
MIRIAM
told him several times about how she needed evidence to believe in something. My point is that him proving everyone wrong would've contradicted this idea. Besides, there's a reason why we only see him with the daugther in the final shot instead of more people.

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I guess that's supposed to be implied, but I would have
loved
to see that scene.
Let's just say that God doesn't believe in me.

I just logged in to make that same post!!!
I would have loved to see him walk through the hospital with his daughter as the camera passes over the dropping mouths of every skeptic, particularly Campbell. Waiting at the end of the hall is Sister Madeline. As Joe hands her to Madeline the two look down at the little girl then both turn to look at the once skeptical staff, not in a "told you so" manner but more of a "now do you believe us" look. The scene fades with the staff giving a gentle smile to acknowledge.
Ok and the non-Hollywood ending would be for Joe to walk down the hall laughing in their faces and saying "And you thought I was batsh!t while little Jeffrey kicks Campbell in the shins and "yeah, you're the one who's batsh!t now."