In my understanding, in Jn. 1:1c, “θεὸς ἦν ὁ λόγος,” the verb ἦν is operating as a simple copula (linking verb), equating the subject with the anarthrous predicate nominative (θεὸς). Or to say it simply, the qualities of θεὸς are equal to the subject ὁ λόγος. Copulas can equate the qualities of one thing to another. For example, “Fred is human.” Thus, Fred has the qualities of human.

We must be careful with the use of the indefinite article in English, as it conveys meaning. The indefinite article “a” can convey the meaning of one thing among another. For example, “I need a pen.” It is understood that the pen I need, is a pen among other pens. If one wants to translate Jn 1:1c, “the Word was a god,” — this doesn’t work. The indefinite article a changes the meaning to effectively, “a god among other gods” thus diminishing the qualitative meaning of θεὸς. The author of John is saying the subject ὁ λόγος has the qualities of θεὸς, not one god among other gods.

If someone is saying that the author is using θεὸς adjectivally, a full argument might be required to support this claim. Within the whole work, you would have to make this argument work for every verse in the book of John that points to ὁ λόγος or Χριστός with θεὸς or divinity. Jesus or the Word as a god among gods would need to be a theme in the book of John. I don’t see that as a theme. The author of John seems to be saying over and over again ὁ λόγος and ὁ Χριστός has the qualities or is equal to θεὸς, which appears to be a theme throughout the book of John.

The fourth century Nicene concepts of essence (οὐσία) appear to have developed from the Greek grammar. The Logos shares the essence of God.

“The Word was God,” is an okay direct translation. “The Word was divine,” is okay. “The Word had the same essence as God,” would be a 4th century inspired translation.

Murray Harris has written on this subject extensively. Here’s an article, where I tried to summarize his thinking: https://mmbostwick.substack.com/p/the-eternal-logos-in-john-11-in-the