Not to speak for Bentley, but I feel one of the most significant differences between Mormonism and Christianity is the doctrine of grace vs works.
(Apologies if I'm misinterpreting things here, but I'm writing this reply based on the assumption that you are saying Mormons aren't Christians due to views on grace vs. works.)
So Catholics aren't Christians, then?
How about Jesus? Was he Christian? Where in the Bible did he *ever* talk about grace as being more important than works? A major point of the parable of the sheep and the goat was to teach that people will be rewarded based on their actions.