Next time a Mormon claims to be a Christian, ask them if, since you are a Christian, it would be accurate to refer to yourself as a Mormon.
There are serveral to ways to discuss this!
1- when I was mainstream Christian and only had the Bible it would not be accurate to call me Bible, or by a name of a book!
2- Since Mormon is a name of a book it would not be accurate to refer to myself or other to call me Mormon. (unless they wanted show contempt or disrespect)
3- The name is The Church of Jesus Christ of Latter Day Saints, those who are cordail us call us LDS.