The Mormon Church's founder and heralding prophet, Joseph Smith, Jr., made the following claim in the last sermon he ever preached:

"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.... he was once a man like us.” (Teachings of Joseph Smith, p.345)

Following are some reasons to reject this belief.

(i) The agreed-upon revelation from God, the Bible, suggests that God is eternal:

(ii) The idea that a finite being can attain omniscience is at least metaphysically impossible.  If someone knows a few things (say, about 5 things) then how could someone obtain a knowledge of an infinite number of things?  It seems that no matter how many things one could come to know, you could always add another fact for someone to know (for example, if you know the value of pi so far as 3.1415926 you could know an additional one (5), and another (3), and another (5), and no matter how many values you come to know in the calculation of pi you could always add another value to it (for it is infinitely calculable and never ends)).  Philosophers of religion concede that the only way any being could come to know the totality of knowledge would be someone who already knew everything in one all-embracing vision.  But this means no one can be made to be omniscient (which the God of the Bible is affirmed to be).

(iii) Regarding the alleged humanity of God, the Bible overtly declares,

(iv) The Bible says that God is a spirit and is invisible:

We have imagined and supposed God was God from all eternity because the Scriptures declare it and reasoning on omniscience demands it.