By now you have probably put the book down or finished it, but thought I'd comment anyway since I just had a similar conversation with a good friend.
The Shack is essentially an allegory. It was written to try to share the author's "faith journey" as he delved more deeply into what he believed about God, life, etc. So if you aren't into thinking abstractly about spiritualism, theology or religion, then this is not a book for you. Don't let it make you feel "stupid." We aren't all interested in the same things.
If you meant that you "don't get" the symbolism - Papa, Jesus and Sarayu are representing the trinity that is part of the belief in many Christian denominations. The different kinds of "personalities" as it were, that God takes on to try to meet us where we are - to try to be what we need him/her/them to be. At least that's how I looked at it.
I have an intelligent well-read friend who read the book and thought it was supposed to be a true story. She was struggling with it. When I told her that it was fiction and explained the origins of it (it's part of a mental journey the author went through, examining his own beliefs and behaviors after having an affair with his wife's best friend) she was livid and wanted to throw it across the room.
Definitely not for everyone. (Just for the record, I liked it.)
Barb