God the Father was a Christ, God the Son is Christ, and God the Holy Ghost will be a Christ

In the Nauvoo period, it is very possible, at least judging by the available sources, that Joseph Smith had reached a theology of the Godhead in which God the Father and all the Gods before him had gone through a mortal probation in a similar manner to Jesus Christ, redeeming a world or worlds through an atonement. According to this theology, Jesus Christ would become like God the Father, and the Holy Ghost would become like Jesus Christ and fulfill the role of Savior in at some future point. The public teaching of this theology happened most significantly in the King Follett Sermon and the Sermon in the Grove, particularly as seen in the notes of one Nauvoo saint, George Laub.

A Godhead of Like Beings

The big reveal in the King Follett Sermon is that God the Father, not only is in the form of a man but also had once lived out a mortal probation, died, and resurrected. In revealing this point, Smith specifically uses Jesus Christ as the type for this and language specifically points to Christ's mortal sacrifice and resurrection. The version of the sermon originally published in the Times and Seasons reads,

Jesus said, as the Father hat power in himself, even so hath the Son power; to do what? why what the Father did, that answer is obvious; in a manner to lay down his body and take it up again. Jesus what are you going to do? To lay down my life, as my Father did, and take it up again.

While, this version can easily be interpreted as saying that Jesus Christ literally did things he saw the father do (i.e. died for others sins, was resurrected), George Laub's notes on the discourse provide a concrete detail. He wrote, 

...Jesus Christ spoke in this manner; I do as my Father before me did. Well what did the Father do? Why he went and took a body and went to redeem a world in the flesh and had power to lay down his life and to take it up again...

http://www.boap.org/LDS/Parallel/1844/7Apr44.html

George Laub's inclusion of the phrase "redeem a world in the flesh" leaves even less ambiguity to the matter. It also parallels with a teaching about the Holy Ghost, which Smith had stated at least half a year earlier. In August 1843, Franklin Richards recorded that JS taught,

Joseph also said that the Holy Ghost is now in a state of Probation which if he should perform in righteousness he may pass through the same or a similar course of things that the Son has.

http://www.boap.org/LDS/Parallel/1843/27Aug43.html

This creates a theology of a consistent Godhead. God the Father was a Christ, God the Son is Christ, and the Holy Ghost will be a Christ. George Laub's notes on the Sermon in the Grove (June 1844),  possibly Smith's second most significant sermon following the King Follett sermon, outline this concept of a consistent Godhead. The relevant portion of Laub's notes read,

But the holy ghost is yet a Spiritual body and waiting to take to himself a body. as the Savior did or as god did or the gods before them took bodies for the Saviour Says the work that my father did do i also & those are the works he took himself a body & then laid down his life that he might take it up again

http://www.boap.org/LDS/Parallel/1844/16Jun44.html

What was likely Smith's conception of the progression of the Gods is laid out clearly here, implying that each God had "laid down his life that he might take it up again."

The Historical Significance of the King Follett Sermon and the Sermon in the Grove

 In weighing the significance of Smith's comments here it's important to remember that the King Follett sermon is not just an important sermon to modern readers interested in Smith's views on the nature of man and God, it was also a huge deal for Smith at the time. It was a big reveal at the equivalent of general conference, through which he was demonstrating his prophethood in the face of criticism and controversy. As is well known, there was a high level controversy during the Nauvoo period surrounding Smith, his practices, and his teachings. Several comments from the King Follett sermon demonstrate that it was intended as a significant reveal to help steady things.

The apostle says this is eternal life, to know God and Jesus Christ, whom he has sent. If any man enquire what kind of a being is God, if he will search diligently his own heart, if the declaration of the apostle by true, he will realize that he has not eternal life, there can be eternal life on no other principle. My first object is, to find out the character of the only wise and true God, and if I should be the man to comprehend God, and explain or convey the principles to your hearts so that the spirit seals it upon you, let every man and woman henceforth put their hand on their mouth and never say any thing against the man of God again; but if I fail, it becomes my duty to renounce all my pretensions to revelations, inspirations, &c., and if all are pretensions to God, they will all be as bad off as I am at any rate. There is not a man but would breathe out an anathema, if they knew I was a false prophet; and some would feel authorized to take away my life.

If any man is authorized to take away my life, who says I am a false teacher: then upon the same principle am I authorized to take away the life of every false teacher, and where would be the end of blood, and who would not be the sufferer. But no man is authorized to take away life in consequence of their religion; which all laws and governments ought to tolerate, right or wrong. If I show verily, that I have the truth of God, and show that ninety-nine out of a hundred are false teachers, while they pretend to hold the keys of God, and to kill them because they are false teachers, it would deluge the whole world with blood.

...the Father of us all dwelt on an earth the same as Jesus Christ himself did, and I will show it from the Bible. I wish I had the trump of a arch angel, I could tell the story in such a manner that persecution would cease forever;

...no man knows my history; I cannot tell it. I shall never undertake it; if I had not experienced what I have, I should not have known it myself. I never did harm any man since I have been born in the world. My voice is always for peace, I cannot lie down until all my work is finished. I never think any evil, nor any thing to the harm of my fellow man.--- When I am called at the trump of the ark-angel, and weighed in the balance, you will all know me then. I add no more. God bless you all. Amen.

The Sermon in the Grove is a follow up and extension of the King Follett sermon, and again it is meant as a significant response to criticism and controversy. It is a public sermon at the grounds near the temple. Hear are a couple of pertinent quotes:

Now, you know that of late some malicious and corrupt men have sprung up and apostatized from the Church of Jesus Christ of Latter-day Saints, and they declare that the Prophet believes in a plurality of Gods, and, lo and behold! we have discovered a very great secret, they cry -- "The Prophet says there are many Gods, and this proves that he has fallen."  It has been my intention for a long time to take up this subject and lay it clearly before the people, and show what my faith is in relation to this interesting matter.

...I testify again, as the Lord lives, God never will acknowledge any traitors or apostates. Any man who will betray the Catholics will betray you; and if he will betray me, he will betray you. All men are liars who say they are of the true Church without the revelations of Jesus Christ and the Priesthood of Melchizedek, which is after the order of the Son of God.

It is in the order of heavenly things that God should always send a new dispensation into the world when men have apostatized from the truth and lost the priesthood, but when men come out and build upon other men's foundations, they do it on their own responsibility, without authority from God; and when the floods come and the winds blow, their foundations will be found to be sand, and their whole fabric will crumble to dust.

Did I build on any other man's foundation? I have got all the truth which the Christian world possessed, and an independent revelation in the bargain, and God will bear me off triumphant. I will drop this subject. I wish I could speak for three or four hours; but it is not expedient on account of the rain; I would still go on, and show you proof upon proofs; all the Bible is equal in support of this doctrine, one part as another.

It is in this context of reasserting his prophethood that Smith taught this exciting new doctrine. He obviously considered it a big deal. It should also be noted that the earlier comment on the Holy Ghost recorded by Franklin Richards in 1843 suggests that Smith had been developing this doctrine for some time. If he had lived longer, it is possible that this theology would have been written down or ritualized, similar to Smith's earlier pattern of doctrinal development through personal revelation and exploration followed by giving the concepts a more concrete form in public revelation or ritual.