While I didn’t see the Book of Mormon musical, I do find the Mormon religion story rather funny: After Jesus died, but before he went to heaven, Jesus stopped in the United States. Mormons know this to be true because the story was chiseled on gold plates, and buried in Palmyra, New York. In 1827, the angel Moroni led Joseph Smith to the gold plates and a magic stone. When Smith put the magic stone into his hat and buried his face in the hat, he was able to translate the gold plates from Egyptian hieroglyphics into English.
I don’t think it offensive to ask Mormons if they believe this story. I don’t think it offensive to ask Catholics if they literally believe each Sunday that they are eating the body and drinking the blood of someone who died and rose from the dead. I also don’t think it offensive to ask Christians, Jews and Muslims if they believe in talking snakes, or a 600-year old man who gathered pairs of all animals in the world and put them in an ark he built, after which he watched everyone except his family and animals drown in a flood.
Some in each religion will say they believe the myths in their own religions, while others will laugh them off as made-up fairy tales. Some will take offense when anyone says their fables sound preposterous, while at the same time finding preposterous the fables in other religions.
What is humorous or offensive is in the eyes of the beholder. I don’t distinguish religious satire from political satire or other forms of social satire. I find all the stories above funny, and I’m not the least bothered by someone who laughs at my lack of belief in supernatural deities or an afterlife. I take it as an opening for dialog.
People I like most don’t take themselves too seriously. How religious people react to humor is often far more damning to the religion than the humor, itself. Muslims could have treated the Danish cartoons of Muhammad as a teachable moment about their faith, or protested peacefully. Claims that Islam is a peaceful religion were not hurt by the cartoons, but by the ensuing violent reaction to them.