As Man Now Is, God Once Was: Did God the Father Ever Sin?

The Noble and Great Ones by godwinthescribe

So the idea that there is a progression from humans into gods/becoming like God came about in the day of Joseph Smith, showing a pattern that would happen often. Someone would come to Joseph with a question or suggestion, he'd embrace it, and then more of the idea would be fleshed out over time as the need for knowledge related to the subject would become necessary for the Saints as a whole. This theology is an example of that pattern.

Here's a brief chronology. And here are the source texts of the King Follett discourse from the Joseph Smith Papers.

The King Follett Discourse is where I'm going to take the core of my answer from because of the explanation Joseph Smith gives in it for how this progression works.

Lorenzo Snow was the first one to approach Joseph with this idea that "As man now is, God once was; as God is now, man may become."

Joseph then went on to teach in the King Follett sermon (which was at a funeral) this same idea, and that it could be reasoned out with Scripture.

There's nothing else beyond those limited comments that have ever been revealed since then. So what I'm giving is pure speculation, but it's based on the comments Joseph made.

Joseph said that in everything Jesus did, he was doing what he had seen the Father do. Joseph then pointed to the council in Heaven and revealed what has become our beliefs about the preexistence. So in the very act of becoming the Messiah, atoning for humanity and bridging them to God by being the demands of divine law, Joseph is suggesting that Jesus was doing something in the act of atonement that has been done before.

If we take this as truth, it suggests one of two origins for God the Father:

1. He was an imperfect human in need of atonement, same as us, and that atonement was provided for by someone else who was the equivalent of Jesus to him.
2.  God our Father WAS a previous Messiah/Christ who atoned for his brothers and sisters, and Jesus was chosen to follow that pattern for us. This would mean that God the Father had to meet the same perfect standards that Jesus did in being sinless to perform this sacrifice. In which case, he would've been capable of sin but resisted all temptation for his entire mortal life. 
   

Based on what Joseph explains in the King Follett Discourse, I personally lean towards the second. I could see this pattern of the Firstborn atoning for the siblings in the divine family being the order that Godhood follows, that Christ is a priesthood office and atonement is an ordinance that has redeemed many before us and will be the pattern of how redemption works into all eternity.

But all of that is pure speculation. We could end up with an answer to this that says something else entirely. I will point out that I've only ever had one other conversation about this with anyone in the Church in almost 18 years. It was with one of the nephews of D. Todd Christofferson when we were in the same student ward together at BYU. I walked into the middle of a very intense conversation between him and some other guys in my ward about this. They got to the notion of "Grandfather God" and he could see the 404 Error screen all over my face. He was very kind in telling me none of this was important for me to worry about and they were just messing around and speculating. You have to be a theology and Religious Deep Lore nerd to even end up here. 

"I can barely handle worshiping one God. I don't need another," is what I said then. And in the end, I think I stand by that.