The brain glitch is always available as a backup explanation, and they certainly do happen (especially in schizophrenics etc.) But if I had an angel come down to talk to me, I would probably believe it.
FWIW, I've had the experience of a Presence manifesting itself to talk to me. The most likely explanation of that experience is a brain glitch. I'm not sure why I ought to consider that a "backup" explanation.
How would you tell the difference? Also see this classic by another LWer.
7JoshuaZ
And if it were a demon? A ghost? A fairy? A Greek deity? If these are different, why are they different? What about an angel that 's from another religion?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.