Biblical Christianity is monotheistic; Mormon theology is polytheistic.
Priesthood in the New Testament and in Mormonism
In the New Testament, Jesus is our only high priest; in Mormonism, every qualified adult Mormon man can be a high priest.
The Temple in the New Testament versus the Mormon Temples
Jesus announced the end of temple religion; Mormonism has invented a new temple religion.
The Biblical and Mormon Gospels
The biblical gospel is salvation by grace alone to eternal life as glorified creatures; the Mormon gospel is exaltation to godhood through grace and works.
Biblical Prophets and the Prophet Joseph Smith
The biblical prophets pass their own tests, but Joseph Smith does not.