It has long been debated whether the translation of "a god" in Jo 1:1c is a legitimate rendering of what John the monotheist had in mind. Scholars, such as Julius Mantey have been misunderstood either for supporting or denying its efficacy.
1. It must be accepted that both uses of the word in their respective cases refer to the same concept. In other words, God, or Deity.
2. The word God, in this context cannot be conveniently nuanced by contradictory applications, and thus altering the meanings, one referring to Deity and the other not. And unfortunately that is what the use of the rendering "a god" does. It makes the phrase "with God" in Jo 1:1b mean God as Deity, and the rendering of "a god" of Jo 1:1c as something less than God, or by contextual semantics something that is "non-God".
3. Thus the rendering "a" god is quite acceptable if Jo 1:1b was made to say "And the Word was with "The" god. But it is not so acceptable if the renderings are said to be: "the Word was with God... and the word was a god" . By using this convenient subterfuge one is tilting the balance in favour of an Arian understanding of the text when it is in fact teaching Monotheism. The kind of Monotheism being declared is in debate, not that the Trinitarian form is wrong and the Monad form is the only one permissible.
4. Harner helped revolutionize our understanding of this grammatical construction and thus went farther than what Colwell did. Colwell was concerned about whether Jo 1:1c made θεος a definite or an indefinite noun. Harner showed that it was neither.
5. Thus this construction is called "The Qualitative Nominative" or "Predicate Nominative". The NWT faces up to one such text in the same grammatical construction at Lu 20:38, where they say of the Father that He is "a God".
6. If the Father can be called "a God" in a construction that is exactly the same as that of Jo 1:1c, then one would at least be satisfied if the NWT also called The Word "A God" at that latter verse. This would not be theologically applicable but grammatically permissible