Social media is filled with the righteous indignation of the Mormons---who claim to be the only true Church, all other churches are corrupt, their creeds an abomination, their ministers the hirelings of Satan---that they, the Mormons, are not really Christians. It's just a tad ironic, but it does play on the ignorance of most LDS, and non-LDS, about LDS history and teachings.
Their current modus operandi is to use the phrase "creedal Christians" and try to make the issue post-biblical. Here's the problem. The fundamental distinction between Christianity and Mormonism goes back to about 1400 BC, not 400 AD. Here is the foundational contrast:
Before the mountains were born,
Or Your brought forth the earth and the world,
Even from everlasting to everlasting,
You are God. (Psalm 90:2)
...for I am going to tell you how God came to be God. We have imagined and supposed that God was God from all eternity. I will refute that idea, and take away the veil, so that you may see.
Joseph Smith, 1844
Now, two quick things: 1) Psalm 90 is a song of Moses, hence the early date. 2) The King Follett Funeral Discourse is not LDS Scripture. But it is the most often cited sermon of Joseph Smith by the leaders of the LDS Church itself, and is one of two foundational sermons preached within the last months of his life.
Hence, the issue is stark and clear. Monotheism vs. polytheism, one transcendent Creator God vs. an exalted man from another planet. Just a reminder from Brigham Young as to how Smith's words were interpreted by those who heard him initially:
“Mankind are here because they are the offspring of parents who were first brought here from another earth, and were enabled to propagate their species, and they were commanded to multiply and replenish the earth. How many Gods there are, I do not know. But there never was a time when there were not Gods and worlds, and when men were not passing through the same ordeals that we are now passing through. That course has been from all eternity, and it is and will be to all eternity.”
Brigham Young, JD 7:333, August 28, 1859.