Do the doctrines of Mormonism really differ that much from Christianity?
Yes, very much so. Besides what we have already seen concerning the Bible and God, there are many practices that Mormons have that differ greatly from what Christians practice. Obviously, Mormons have a lot of similar beliefs as Christians, but they tend to put their own spin on it. As I mentioned with Jesus Christ, earlier, we saw that much of it was similar, but some crucial facts was drastically changed (enough to change the person Mormons believe in). Every Mormon doctrine is somehow spoken of in some way in the Bible. LDS theologians merely took those things and made them doctrines. Thus the differences can be from changing existing doctrines slightly or drastically, or accepting doctrines that the Bible strictly says not to (like following after false prophets).
