First, let me ask mainstream Christians a question, since you believe God is unchanging as well. Did Jesus change when He was born as a mortal? Do you think of that as God changing? Of course not. Yes, Jesus physically changed, but not in the way those verses are saying. God is unchanging is His moral nature.

Christ changed numerous times throughout his life. Christians have maintained that Christ was born with two natures. A human-created nature that changes and develops over time. He also possesses a divine-uncreated nature that doesn’t change and isn’t subject to time. This is called the Hypostatic Union. Furthermore, if statements about God being unchanging are limited to his moral character, then it still seems to rule out being human. That is because humans are mutable by nature and morally capable of evil. Especially, if they have Libertarian Freewill. It seems like at any moment the character of a person can change.

With that in mind, let’s ask the question of why Latter-day Saints believe God was once a man. John 5:19 says:

“Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.”

To summarize, we believe that the Father was once a man because we know the Son was once a man, and this verse clearly states that anything the Son does is something He’s seen the Father do. Granted, I’m not saying that if Jesus ate eggs for breakfast, then He must have seen the Father eat eggs for breakfast. But the major points of Jesus’s ministry? This would mean the Father had once lived a life similar to Jesus’s. This, along with a few other verses and modern revelation, is where we get the idea that the Father was once a man. I’d like to point out here that just because we say the Father was once a man doesn’t mean He was once a sinful man. The argument from John 5:19 doesn’t work unless the Father lived a sinless life like Christ did.