"In the beginning was the Word, and the Word was with God, and the
Word was God."
Ἐν ἀρχῇ ἦν ὁ λόγος καὶ ὁ λόγος ἦν πρὸς τὸν θεόν καὶ θεὸς ἦν ὁ λόγος
A summary of the interpretation I think is most plausible is
"In the [new] beginning [i.e., Jesus' ministry] was the Word [a title for
the Christ, because he reflects God's word], and the Word was with [as Moses was with, 'pros ton theon'] God
[i.e., the Father], and the Word was [in the sense of agency and
image] God [where God again = the Father]."
What are our main interpretive questions?
1. Which 'beginning' (arche)?
2. Who or what is the Word (logos)?
3. What does it mean for the Word to be 'with' God (pros ton theon)?
4. What does the first use of 'God' (theon) mean? Is this just the Father? Or is it God inclusive of 3 persons?
5. What is meant by 'was' (en) in 'the Word was God'?
6. What does the second use of 'God' (theos) mean? Is it the same as the first use?
Each of these points is open to contention, among both Unitarians and Trinitarians.
Here I want to argue for an interpretation which has been given recently by Bill Schlegel, and which is broadly Socinian.
1. Which 'beginning' (arche)?
The phrase 'in the beginning' is not unique in the NT to John's prologue. It is used 2 other times, in one case referring to the beginning of Paul's missionary work, in the other the beginning of the Church at Pentecost. Similarly, 2 other Gospels use 'beginning' (arche) at the very beginning of their Gospels (Mark 1:1, Luke 1:2), and in each case refer to the beginning of Jesus' human life or ministry.
So it is obvious that simply noting the similarity to Genesis 1:1 is not enough to mean that the beginning referred to is primordial spacetime, or something like that.
If we use our cue from the other Gospels, the obvious candidate for the 'beginning' here is the beginning of Jesus' ministry.
Whether 'beginning' is understood as the new beginning (related to Jesus' human life, ministry, and ascension, say) or the old beginning (related to Genesis 1:1) greatly affects the rest of the reading of John's prologue.
On the interpretation I am following here, the beginning is the new beginning, and that lends itself to understanding the 'Word' as the human, Jesus, although that is not necessary.
This is explored in depth in an answer to the question What are the arguments in favour of the 'beginning' at John 1:1 being the new beginning?.
2. Who or what is the Word?
Taken in a void, this is not very clear.
Although John's Gospel doesn't directly give Jesus the title of 'the Word' (outside of possibly John 1:1), unlike 'the Light', 'the Bread of Life', and so on, Revelation 19:13 does.
"His name is The Word of God."
The identity of the Word is contentious within Unitarian circles, but I think the development of the passage from 1:1 to 1:14, and in particular 1:10 and 1:14b, leads to the most straightforward answer grammatically being that 'the Word' = a title for Jesus.
Many Unitarians don't like this, because they believe that if the Word = Jesus and 'the Word was God', then it would mean Jesus is God in the same sense the Father is God. So they resist, because of broad contextual reasons (i.e., scriptural reasons to think Jesus is not God in the sense the Father is God). If, though, there's another more plausible reading of 'the Word was God', then many of the broad contextual reasons Unitarians have for denying the Word = Jesus fall away. We will discuss this later, but briefly, the Word is God in the sense of agency (similar to how Moses 'was' God to Aaron and Pharaoh in the OT).
So, on this interpretation, the Word = a title for Jesus, due to his being a perfect conduit for God's word, a theme repeated again and again in the Gospel of John.
3. What does it mean for the Word to be 'with' God (pros ton theon)?
The understanding of 'with' here will depend to a large degree on our understanding of what the Word is. Is the word a power of God? Or is the word a title for a person?
We go with the second option at 2. above. So when we talk about 'with', it is about this person being 'with' God. How so?
The Greek phrase 'pros ton theon' is used repeatedly to describe Moses' relationship with God in the Septuagint (Exodus 8:29, 8:30, 10:17, 10:18, 18:19, 19:8, 24:2, 32:30), the Greek translation of the OT. As Bill Schlegel discusses in depth in his article John 1:1 is parallel to the man Moses?,
Moses made mediatorial supplication pros ton Theon. Moses represented
the people pros ton Theon. Moses only came consistently into unique
spatial proximity pros ton Theon. For a Greek Old Testament reader,
the coming into or being in the position pros ton Theon described
neither a second divine figure nor an abstract attribute like Wisdom.
It was the human being, the man Moses, who was pros ton Theon.
So John 1:1's 'with' means that Jesus, like Moses but to a far greater extent, is in a mediating or interceding role and in relational proximity to God (a point reiterated at John 1:18, the final verse of the prologue, which despite having textual variants and semantic debates, is clear about Jesus being very close to God and making God known).
Not surprisingly given this interpretation of John 1:1's 'with' as relating to Moses, Moses is one of the 2 other humans mentioned in John's prologue (the other being John the Baptist). In particular, Jesus is contrasted with Moses in the prologue, as Jesus is like Moses but greater, at John 1:17.
For the law was given through Moses; grace and truth came through
Jesus Christ.
This contrast to Moses (Jesus > Moses) occurs repeatedly through John's Gospel, and is a major theme. So it makes sense that in his first sentence of his prologue, John uses a phrase that would be associated with Moses for Jesus, the new Moses.
4. What does the first use of 'God' (theon) mean?
This is quite straightforward. God = the Father = Yahweh.
There is no difficult-to-understand theology involving multiple persons in one substance or essence that needs to be posited on this understanding, and therefore no question about the target.
This is also how John seems to use 'God' in the rest of his Gospel (20:28 is discussed below), and explains why Jesus says the Father is the only true God (17:3), and that Jesus himself is a man who has heard things from God (8:40).
5. What is meant by 'was' (en) in 'the Word was God'?
It would be odd if John dropped a theological bomb in 1:1 only to never revisit it in the rest of his Gospel. What is the most obvious correlate to 1:1 in the rest of John's Gospel?
The answer is John 20:28.
Thomas answered and said to Him, "My Lord and my God!"
Thomas here is saying to Jesus "my God." What does Thomas mean? The key to understanding Thomas' statement is in John 14, the previous time Jesus speaks to Thomas. Jesus says "When you see me, you see the Father". When Thomas says 'God' here, he is saying he sees the Father 'through' Jesus. This isn't an ontological identity claim, but an agency identity claim.
This is explored at depth in option 5. in the answer here to the question How do Biblical Unitarians contextually explain Thomas' exclamation at John 20:28?
Just as Jesus is God to Thomas at 20:28, so the Word is God at 1:1c. Similarly, Jesus is God because when you see him, you see the Father (John 14:9). The ontological basis of this that John presents isn't a convoluted Trinitarian theory that John had never heard of, but rather the simpler notion of co-inherence (Jesus is in the Father and the Father in him, John 10:30, 14:11), which is also the relationship between the disciples and Jesus. No Trinitarian thinks that therefore the disciples are God in the same sense that the Father is God.
So, given our answer at 4. above re Moses as 'with God' ('pros ton theon'), do we find that Moses also 'was' God in the OT? That would then link 1:1b and 1:1c together.
Yes, Moses 'was' God to both Aaron (Exodus 4:16, 'shall be God', same verb as 1:1c, 'to be') and Pharaoh (Exodus 7:1, 'made you God') in the OT. These are sometimes glossed in translations with 'as' or 'like' ('as God'), but the 'as' is not in the original. Bill Schlegel discusses this in depth as well in his article John 1:1 is parallel to the man Moses?. Similarly, see also Schlegel's answer regarding Ex. 4:16 in particular, and note the discussion of Keil and Delitzsch in the question.
There is no big metaphysical mystery with Moses - there is no need to add another co-equal person to the 'essence' of God. Instead, it is obvious what is meant - Moses, like Jesus, is a man who 'is' God in the sense that he represents God - to Aaron and Pharaoh. Jesus 'is' God similarly to how Moses 'is' God - but moreso, because Jesus is the new Moses who is even greater than Moses.
This strong sense of agency is applied again and again to Jesus in John's Gospel - as with the other major points above, it is a major theme in the Gospel. Indeed, 'Christ' means one who is appointed by God and rules with God's authority - in other words, is God's agent.
6. What does the second use of 'God' (theos) mean?
It is the same as the first. God = the Father = Yahweh. So Jesus 'is' the Father in the sense of agency or image.
Note that unlike Unitarians, there doesn't seem to be a straightforward way Trinitarians can have a stable assignment of meaning to 'God' in John 1:1.
Conclusion
John' prologue uses condensed, poetic language. Any interpretation of it should be buttressed by larger themes in John's Gospel, and its coherence with the NT more generally. Understanding the material John is drawing on, and his audience would be familiar with, requires looking at the OT first and foremost.
For 1:1 we find a plausible target and sense for key terms in the OT (Moses), this target is buttressed by the prologue itself (1:17 in particular), and repeatedly in the rest of the Gospel.
Jesus, a man chosen by God like Moses, is leading the new exodus. Instead of Moses' leading the ancient Israelites from physical bondage to the geographical promised land, Jesus is leading the new Israelites from spiritual bondage to the much more important promised land of the Kingdom. Jesus is the new Moses, the one Moses himself prophesied about. As John 1:45 makes clear,
“We have found the One Moses wrote about in the Law, the One the
prophets foretold—Jesus of Nazareth, the son of Joseph.”