I'm sure every person on the Internet has seen this before, but I'll post it here anyway, just in case you haven't:
The clip is from a 1985 claymation film called The Adventures of Mark Twain, which I guess saw a few TV broadcasts despite (or perhaps because of) being intensely creepy.
This particular sequence is based on Twain's unfinished novella, The Mysterious Stranger, which is in many ways just as upsetting as the animation. It's also a very interesting meditation on the nature of God and religion, though I'm not sure it's a good story (I guess that's what "unfinished" indicates). Still, Twain was nothing if not a master wordsmith. Go read the novella.
I still very much want to read this story.