Mormonism: Why is it a Cult?

Mormonism is considered to be one of the four major cults. But what makes it a cult? On the surface, it seems that these are pretty valid religions and that they could be considered Christian. But when you look deeper you see how very far from Christianity they really are. Cults disagree with the fundamental doctrines of the Christian faith, such as salvation by grace alone. They also claim to have another authority other than the Bible (e.g. the Book of Mormon.) Cults consider theirs to be the...