The first thing you have to realize when you are talking to a religious person is that they have a built-in persecution complex. They, through years of indoctrination and what I consider effectively brainwashing, have generally come to believe that not only is the rest of the world "unsaved", but that the rest of the world is against them for being a bearer of the "one true faith". The only way to communicate with them on a level resembling true discourse is to remain calm and not become agitated at all. To get angry with them in any way just validates in their mind that they are right and your anger or discomfort or emotion is a test of their faith by Satan. It also tends to push them into "prayer mode" by which I mean the state of mind where they actively try to shut you out completely by reciting prayers or scripture by memory. This is a conditioned reflex that cults/religions instill in the sheep to keep them from thinking for themselves.
I've spent years talking, debating, and arguing with religious people, especially Christians. And Mormons are even worse. Their mythology is like Christian mythology on an Acid trip. Seriously whacked. The Mormon church is even better at brainwashing (IMO) than most other churches, so to argue with a typical Mormon is to waste your time and energy.