"John 1:1 says the Word was 'a god,' not God"

In John 1:1 the last clause has no article before "God" — it is literally "the Word was a god." The New World Translation gets it right; your Bibles hide it.

The missing article does not mean "a." In Greek, a predicate noun that comes before the verb regularly drops the article while staying fully definite or qualitative — this is a well-documented pattern, not an exception invented for this verse. Here it makes the clause qualitative: the Word has the full nature of God. If John had written it with the article, he would have collapsed the Word into the Father and taught modalism; if he had used an indefinite construction, he would have said the Word was one god among others. He chose the wording that says the Word is God by nature and yet not the person of the Father.

That is just Trinitarian bias in the grammar. Plenty of translators disagree.

Test it against John's own usage. Three verses later he says nothing that was made was made without the Word (1:3) — that puts the Word on the Creator's side of the line, not among created things. In the same chapter the Word is the true light, life itself, and "the only God, who is at the Father's side" (1:18). At the end of the Gospel, Thomas addresses the risen Jesus directly as "my Lord and my God," and Jesus accepts it and blesses those who will believe likewise. A Gospel that opens with "a god" would not end there.

But even the early Christians before Nicaea did not all say the Son was fully God.

The ante-Nicene writers who comment on John 1:1 — Irenaeus, Tertullian, Origen, Hippolytus — consistently take it to mean the Word is truly God, one in being with the Father, while personally distinct from him. They argue about how to state the relation, not about whether the Word is divine. The rendering "a god" has no real footing in the Greek grammar, in John's Gospel, or in the people who read it in Greek as their native tongue and were closest to the author.