An informative and helpful comment, Alan. You bring up the important grammatical point that Greek had no indefinite article, so if someone wanted to refer to A god rather than THE god s/he would simply omit the definite article. Alternatively, the omission could also be emphasizing the essential godliness of the Christ. As Zerwick puts it:

The omission of the article shows that the speaker regards the person or thing not so much as this or that person or thing, but rather as such a person or thing, i.e. regards not the individual but rather its nature or quality. Hence it is sometimes stated as a rule that the article is not used with the predicate. In fact, predicates commonly lack the article, but this is not in virtue of any rule about predicates in particular, but in virtue of the universal rule; for in the nature of things the predicate commonly refers not to an individual or individuals as such, but to the class to which the subject belongs, to the nature or quality predicated of the subject; e.g., John 1:1 "and god was the Word," which attributes to the Word the divine nature ("the God was the Word," at least in NT usage, would signify personal identity of the Word with the Father, since the latter is "the God").

Zerwick’s book on NT Greek usage was the "bible" in evangelical seminaries in my day — maybe it still is. Zerwick was a scholar at the Pontifical Biblical Institute in the Vatican, so his work spans both the Catholic and the Protestant theological worlds.

I wondered whether the insistence on Christ as THE God in trinitarian theology might have reflected linguistic confusion among early Church scholars. At the time the NT was written Greek rather than Latin was in effect the "lingua franca" of the Empire. Greek was also the language of scholarship, such that even in Rome the educated classes could speak Greek. However, as the Empire expanded Latin became the primary language spoken in the new Western provinces. The Church, always divided to an extent between east and west, split more decisively when the Roman center of the western branch moved decisively to Latin. The western churchmen began relying more heavily on various Latin translations of Greek NT fragments, culminating in Jerome’s consolidated Vulgate in the 5th century. Whereas Greek used a definite article but not the indefinite article, Latin had neither. Imagine the confusion of the early Latinate theologians trying to arrive at a determinative interpretation of John 1:1 having neither "a" nor "the" at their disposal!

However, the eastern branch of the Church, reading the NT in their native Greek, also asserted a trinitarian understanding of God. So even though they probably didn’t read John 1:1 as Biblical proof that Jesus is one in being with Yahweh, they nonetheless asserted the equivalence of three persons in the one God in the same way as the Western church.

Presumably the early Christian theologians were trying to reconcile two seemingly irreconcilable points: there is only THE God, but Christ is ALSO God. In the doctrine of the Trinity they arrived at a synthesis. That they could formulate such a complex understanding of the nature of God testifies either to the intrinsic complexity of the Godhead who reveals Himself in the threefold witness of Scripture, community, and sensus divinitatis, or to the intrinsic complexity of the human mind that through detailed investigation of the Biblical texts, sophisticated reason, and unbridled imaginion could arrive at such a remarkable reconciliation. It remains to be seen whether the emerging church will reconsider the traditional trinitarian understanding, and also whether it will continue to insist on trinitarian belief as one indicator of being a "true" Christian. I supsect that if one were to ask ordinary Catholic or Protestant churchgoers about the Trinity they wouldn’t "get it right" anyway.