You can find the whole series here.
John is unique among the four Gospels in that it speaks of incarnation (John 1). The other Gospels never speak of a previous life for Jesus. In John, Jesus lived with God before mortal life. And everything he says and all his acts conform to what he heard and saw with God in a preexistence (John 5). Joseph Smith makes a huge thing of this, and it forms one of the two pinnacles of the final part of early Mormon teaching. Once you start to think this way, and John does it right from the beginning, it changes everything. The other Gospels make it clear that Jesus is speaking for God, but they never give an explanation of the genesis of that teaching. John does this, and it turns into one of the two pillars of Mormon cosmology.
It’s possible that this incarnation theme is a product of the second epoch of the Gospel, but it’s certainly possible that it represents preaching from Jesus himself, at least the portion we have in John 5. It’s really impossible to say for sure where it began to be a prominent theme in the preaching, before, or after the resurrection. But it’s there and it was influential on the early church and clearly in Mormonism, though the linkage is somewhat hidden. One place it occurs prominently is the Book of Mormon: the Condescension of God. This is the incarnation. And it has John all over the face of it. In the Book of Mormon, Jesus’ previous life is as the Lord Omnipotent. This is an odd phrase, and one that seems almost creedal in its intent. It is in a sense, a forecasting of Matthew’s dual Jesus: Davidic Messiah, and Son of God. Of course, the Book of Mormon doesn’t spend a lot of time with David. He’s not an admirable character in spots where he appears (and the temple is largely irrelevant for Mormon), in fact the only time he appears marginally important is in Isaiah quotations. He’s engaged in serious abomination for Jacob. This is not Matthew’s view at all, but it is certainly Johannine in terms of rank.
This incarnation teaching seems closely connected to the Wisdom Literature of the Old Testament, and it’s not hard to give some examples that point to this. First, the Gospels portray Jesus as a person of divine wisdom. The Wisdom Literature include books like Proverbs, Ecclesiastes, and parallel to these are a lot of parables, wise sayings, and so on in the Gospels. That aspect of Wisdom Literature is reflected in the Jesus of the Synoptics (Matthew, Mark, Luke). Jews who heard Jesus, saw in him this Wisdom tradition. There is also another aspect of Wisdom Literature, some in Proverbs, some in Apocryphal books like Ecclesiasticus or Sirach, and the Wisdom of Solomon.[2] This other aspect of wisdom is more abstract, and often portrayed as personification. The personification nods to Wisdom (a feminine noun in Hebrew) as the first of the creation or emanation (Book of the Wisdom of Solomon says emanation) from God (and this is important for John). God makes all things as Wisdom dictates. In Sirach you have episodes where Wisdom speaks of herself, her relation to God, her role in creation, her love of creation, she wants to guide the creatures of God, how she comes down to live among people, she is represented in the Law (of Moses) and she nourishes the people.
Sirach chapter 24 tells of Wisdom speaking in the heavenly assembly (Grand Council?), how she came from God’s mouth, lived in the highest heaven, encompassed the whole of heaven, She was over the sea, and to every people, she dictates. “Then the creator of all things gave me a command” and this is found in John in reference to Jesus, God commands him. “My creator chose the place for my tent where I would dwell.” In John 1: the Word came down and dwelt among us (literally set up a tent among us). “And God said, make your dwelling in Jacob, in Israel receive your inheritance.” “Before the ages, in the beginning he created me, and for all ages, I shall not cease to be.” (John: Jesus existed with God from the beginning and will never cease to exist.) “In the holy tent, I ministered before him, and I was established in Zion.” (She was in heaven then came down to Jerusalem, because she is the Law, and that centers in the temple.) “Like the vine, I bud forth the lights.” (John: I am the vine, you are the branches.) The Wisdom Literature has a very prominent presence in John. These wisdom books had influence in culture outside Judaism, Sirach was translated into Greek, the Book of Wisdom was written in Greek and it doubles back into the tradition later on.
John also adds another twist to Christian eschatology. The other Gospels emphasize that Jesus will return from heaven, people will see his true glory, there will be the judgement, and eternal life will come to the chosen. John turns this on its head a bit. The idea that Jesus came from heaven reads all of those things as a part of the ministry—offering eternal life, second comforter, it’s salvific in the present. In fact Joseph Smith draws heavily on this in his theology of assurance, all of which finds some basis in John. Joseph Smith’s sermon cycle of 1843 on election is primal in his teaching of polygamy and much of its related theological diaspora finds a connection in John.
Next week: John and eternal life.
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[2] See R. H. Charles, The Apocrypha and Pseudepigrapha of the Old Testament, 268ff.
A systematic treatment of intertexuality with regard to John’s Prologue and Mormon scripture, something I will approach very briefly as I go on with this series, is found in Nicholas J. Frederick, “Line Within Line: An Intertextual Analysis of Mormon Scripture and the Prologue of the Gospel of John” (Ph.D. Dissertation, Claremont Graduate University, 2013). For a discussion of some of the issues that overlay much of what I’m about in this series, see here. If you have access to a university library, Frederick’s dissertation is available through proquest. Others can also get the dissertation through proquest for a fee. While I do not appeal to the formal methods, arguments, and conclusions of Frederick (at least intentionally), it is a valuable study for the interested.
By Common Consent, a Mormon Blog 
I’ve been trying to make sense of the JST of John 1:1 “In the beginning was the gospel preached through the Son. And the gospel was the word, and the word was with the Son, and the Son was with God, and the Son was of God.”
By v 14, JST gets back to KJV language: “And the same word was made flesh, and dwelt among us, and we beheld his glory, the glory as of the Only Begotten of the Father, full of grace and truth”.
How does Word = Gospel play into Word = Jesus within the theme developed here?
HDP, I didn’t bring the JST into this much, since I think it breaks up some of the unity of the text. I think JST John 1:1 is an allusion to the JST Genesis (Book of Moses) myth, and perhaps a cautionary on some version of Trinitarian cosmology. But, as you point out, the translation seems to rotate back to the front of the Prologue. I confess that I don’t know the LDS commentary literature well, and so there may be some deeper answer, but I chalk it up to some kind of “waiting for copy edits” thing.