In his book All in the Name, Mark McNeil discusses a word game that his old Oneness Pentecostal pastor would play in an effort to "gotcha" Trinitarians over John 1:1:
My Oneness pastor would often explain this text by asking, "Who is your God?" If you answered, "The Trinity," he then read John 1:1, inserting the word "the Trinity" each time the word "God" appears... This reading is [apparently] problematic for Trinitarians since it suggests the Word is something other than or outside of the Trinity, yet at the same time, identical to the Trinity. If we say, on the other hand, that the Word (or Son) was with the Father thereby interpreting the first use of "God" as a reference to the Father, he then suggested [that the Word or Son was the Father]....The Oneness solution is that the Word is simply an aspect of God, the divine plan or thought, and therefore can be said, rather poetically, to be with God but, at the same time, is, as an aspect of God, God. {pp. 65-66}
There is a problem here. I should say that, before addressing my thought on this, Mr. McNeil does a fine job of answering this in his book. I would add this comment: what is sauce for the goose becomes sauce for the gander. That is to say, I notice some equivocation in the Oneness explanation which is not allowed for the Trinitarians. After all, I could simply substitute "Jesus" for "God", as the Oneness Pentecostals do in claiming that Jesus is the one true name of the one true God. Now the Word is with (outside of, per their reading of "The Trinity") Jesus, and the Word is Jesus. If we allow the "poetic" reading, that "The Word" is Jesus divine plan, and yet that this divine plan is in some sense Jesus (e.g. we pivot to, say, the Incarnation). This is a neat interpretation, but it requires use to ask, as a former US President asked, what the meaning of the word "is" is.
As C.S. Lewis notes, there are different uses of the word "is," and they are not all equivalent. To say that God is love is not the same as to same that love is God--and indeed, the latter interpretation can have some rather disastrous effects. I would agree with the Oneness statement that the Incarnation "is" God, in that in Jesus Christ we have two natures--one human, one divine--which are united. Thus, Jesus is God. However, it is an entirely different use of the word "is" to say that the Incarnation was with God from the beginning, or to say that God was ("is" past tense) incarnate with a body and human nature from the beginning* (and, we know that Jesus' body changed in time, as He was born, grew, was tormented, dies, was resurrected). If we mean instead that this plan "is" with (outside of) God from the beginning, but also "is" God in the second sense (that is, God is present in the Incarnation), we have to the ask why "The Word" (divine plan) took on flesh and dwelt among us, as opposed to simply saying that "God took on flesh and dwelt among us." In some sense, this latter interpretation is what the Arians and semi-Arians were claiming.
It follows that there is no reason to force an interpretation which says that "with" means outside of (the Trinity), or alternatively that the second use of God must be identified with the first. After all, the Oneness solution requires that we equivocate on the meaning of "is." We could as easily read this to mean, "The Word was with [the other Persons of] the Trinity, and the Word was [a Person of] the Trinity," or "The Word was with God [the Father], and the Word was God [the Son]."
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*We would say here that God united his divine nature to human nature: rather than gaining or adding to His nature a human one, he instead added/united His divine nature to a human one so that a man has now also a divine nature.
