Mormonism is not Christianity. It is fundamentally different. While it differs in hundreds of minor details, what is most important to us is that it differs in the most definitional and fundamentalaspects from the Christian faith. Christianity worships one God. Christianity knows that there is one God who has eternally been God and is the Creator of all things exhaustively. Everything else in the Christian faith, from the Trinity to the atonement to the gospel to the resurrection, flows from and is defined by, this fundamental truth. And it is just here that we cannot help but hear the words of Joseph Smith when, addressing the idea that God has been God from all eternity, he said, “I will refute that idea, and take away the veil, so that you may see.” Smith cannot refute God’s truth, and of course, he failed to do so. But the result is beyond question: Mormonism, the system he founded, is not Christianity. It is a false religion with a false god. And if Mormonism is a false religion, then the believing Mormon is deceived and lost. There can be no other honest answer.
And if we are truly concerned with pleasing God and rightly handling His truth, we will have to go one step further. As we saw in Isaiah 41:24, choosing to worship a false god is a fearful choice. The true God takes His sole rights to worship very seriously. When speaking to the idols, God says, He who chooses you is an abomination (to-evah). The teaching that God was once a man, and that men can become gods, involves simple idolatry, for it makes of God something that He is not, and lowers Him to the level of His creation. There is no salvation in such a god, and those worshipping this god are under the wrath of the true and living god.
—Is the Mormon My Brother? pp. 169-170.