Chris Beam previews it here:
What Parker and Stone do isn't religion-bashing. It's religion-teasing. And it's born more from fascination than disdain. "I'm an atheist that admires and likes religion," Stone told me in an interview. He describes the new musical as "an atheist's love letter to religion." If you had to classify Parker and Stone's world view, you might call it Hobbesian absurdism.
In the universe they've created, random, terrible things happen with no explanation. It's no coincidence that South Park's most famous line is "Oh my God, they killed Kenny!"/ "You bastards!"—in response to the frequent death of Kenny McCormick—with no explanation of who "they" are. Parker and Stone's Book of Mormon has a similarly bleak perspective. When the two missionaries arrive in Uganda, they find the natives singing what sounds like an uplifting "Hakuna Matata"-like spiritual. It turns out what they're chanting—assa dega ebo aye—actually means "Fuck you, God." The rest of the musical chronicles the missionaries' attempt to reconcile their faith with this place that God appears to have forgotten.
Religion is good dramatic fodder for a Broadway show. Young believers are strong-willed, forward-moving, confident of their place in the universe—just the kind of hubris that makes for a good slapped-in-the-face-by-reality story. Adding to Parker and Stone's fascination is the fact that Mormonism is itself a young religion. "It's like Darwin's finches of religion—we can watch it evolve," says Stone.