I've shared the following thoughts about Joseph's King Follet Sermon with others elsewhere. Admittedly, they're not that original. But they are, nevertheless, now my thoughts. These are the words I have chosen to express what not only feels right, but what makes the most sense to me after having read Joseph in context.
For those who haven't read the sermon, the most common format is the amalgamated version (which is accessible in two parts here and here. The non-amalgamated scribe notes (which I find more valuable) can be found here). I think it's critical to try to step away from years of implicit assumptions and interpretations about it and and look at what Joseph was explicitly teaching. I've taken a great interest in this sermon and topic. By most accounts, Joseph teaches some pretty radical doctrine (at least as far as the traditional Christian world is concerned), but that's one of the things I love most about Joseph Smith!
First off, I think too many people pick one or two quotes out of context and interpret Joseph to be teaching that God was once not God, but was once merely a man--even sinful--exactly like us. And this despite the fact that Christ was God before he took upon himself flesh and was also sinless throughout his mortality. Not only do I believe that this is a significant misinterpretation of Joseph Smith, but I also fail to see how that teaching would have been any comfort for a funeral sermon. (Remember that the occasion for the sermon was the sudden and accidental death of Joseph's close friend, King Follet). Rather, here's how I personally understand Joseph Smith. He took this occasion to declare a "great secret"--that God the Father once had a mortal experience. To paraphrase Joseph, "we've supposed that God has always been right where he is, eternally dwelling in His heaven, without any changes. But that's actually not the case! He too came to an earth and took upon himself mortality!" That is the great secret. Not that he hasn't always been God, but that He too had a mortal experience. God our Father understands us perfectly, even in the midst of trials and death, since he's been through it too!
Nevertheless, Joseph was not teaching that the Father's mortal experience was undertaken exactly like ours (that is, not as God), but rather "the same as Jesus Christ". Jesus Christ was also "a man", but like the Father, He was God while he had His mortal experience. We, on the other hand, are mere mortals, and clearly there is a difference between our mortal experience and that of Christ's, not the least of which was the fact that he was perfect and that he never sinned. He was God, I am not. Therefore, the "great secret" wasn't that God was once not God, but that God is in the form of a man (albeit a supremely exalted one), and that he dwelt on an earth "the same as Jesus Christ Himself did". The real revolutionary teaching, of course, is that we're of the same kind or species as God! When Joseph says that God is a man like us, he's teaching that God isn't some "substance", but rather that humankind and the divine are of the same kind/species and are not permanently separate, despite the traditional Christian belief about the Creator/creature divide.
I reject the interpretation which holds that God the Father was once merely a man and then grew into becoming God. I'm sure that many people (including Church leaders) have probably made that assumption, and still do, but I think it's the weaker argument. For me, that's not only unscriptural but it just was not what Joseph was teaching as I go back and read the actual text(s). One thing that seems very clear to me is that Joseph did not mean to contradict scripture--and he doesn't. He never said he was going to refute the Bible, but rather, that he was going to show this teaching from the Bible! The teaching was that the Son can do nothing but what he sees the Father do. Therefore, the Father took on mortality, became embodied, and experienced mortality the same as Jesus did (as a perfect, sinless, human-God) with the power to take up his life again. This is a power we clearly do not share.
Some people assume too much and think that Joseph implied things he never actually taught. For example, I've seen some argue that if the foregoing is correct, and the Father experienced mortality more in line with how the Son/Jesus experienced mortality, then that must mean He too was a Savior and performed an atonement. But again, that's just not an explicit teaching of Joseph Smith, and people are on thin doctrinal ice by making those assumptions. Joseph simply teaches that the Father had power in himself "to lay down his body and take it up again". I still believe in only one "infinite atonement". I've come to believe that if the Father can create multiple worlds by the power of His Son (Moses 1:33), then he would also redeem those worlds by the power of His Son. Otherwise, we'd have to reinterpret what "infinite" means in "infinite atonement".
The King Follet sermon goes on to include many more marvelous teachings from a prophet of God, including the radical rejection of the traditional doctrine of creatio ex nihilo (or creation out of nothing). I find it so much easier to appreciate the sermon and the prophet by not getting caught up with troublesome interpretations that don't ring true. Too many people bring previous assumptions to the text, or even the Lorenzo Snow couplet, and read it through that filter, rather than looking at what Joseph Smith was actually teaching.
For the record, the Lorenzo Snow couplet states that "As man is God once was, as God is man may become". Both the Father and the Son can both correctly be referred to as God, so if we read this couplet as referring to God the Father, I think we need to remember what Joseph taught about God's mortal experience and the divine power he had while a mortal. We could gain another appreciation for it by interchanging "Jesus Christ" for "God". We know that He too was/is God--not only the Son of God, but God the Son--or as the Book of Mormon title page says, "the Eternal God". Thus, as man now is, Christ once was. As Christ now is, man may become. What does that mean? What does it not mean? If we drop our previously held assumptions, things become a lot clearer. And that's how I also view the Snow couplet, whether we're talking about the Father or the Son, they were once as man is now (experiencing mortality) and we can become divine too! That is the heart of the gospel!