θεός is anarthrous (i.e. lacking the article "the") in John 1:1c. In Greek, to emphasise that the predicate noun θεός is qualitative, the anarthrous θεός is placed before the verb: θεὸς ἦν ὁ λόγος. The English translation ''the Word was a god'' is not an accurate translation of the Greek because it lacks the emphasis on θεός as qualitative.
In English, to express the predicate noun as qualitative the use of the indefinite article is required ('a man'', John 10:33). However, when the goal is to emphasise the predicate noun as qualitative, we don't use an article (either definite or indefinite). For instance, to emphasise that the predicate noun 'god' is qualitative, we use the word "god" by itself (''the Word was god'', not ''a god'' or ''the god''), as in the case of κύριος in Mark 2:28 ''(the son of man is Lord, not a Lord'' or ''the lord''). Majority of English translations accurately translated the anarthrous noun θεός in John 1:1c: ''the Word was God''.
The grammatical function of the anarthrous pre-verbal noun in Greek is to emphasise its substantive meaning upon the subject. The presence of the predicate noun before the verb puts emphasis on the predicate noun as qualitative. For instance, in John 1:14, the predicate noun also occurs before the subject. It puts emphasis on the nominative σαρχ as qualitative. The Word became neither ''a'' flesh nor ''the'' flesh but simply (as the grammar requires) became ''flesh''.
In English, the predicate occurs after the verb but in Greek, there are cases where the predicate occurs before the verb. The function of this Greek word order is to emphasise the predicate nominative (i.e. predicate noun) as qualitative. In English, to emphasise the noun as qualitative, we don't use articles (either indefinite or definite). The grammar requires that the noun alone be used in this case.
The anarthrous noun θεός is the predicate noun in John 1:1c. John put the predicate noun θεός before the subject to emphasise that θεός is qualitative, not to transform θεός into an adjective. θεός does not have the meaning of θειος (a person or thing "of divine quality"; "of god"; "god-like'). The noun θεος retains its substantive meaning (i.e. its meaning as a noun).
In English, to convey that the predicate noun ''god'' is qualitative, the indefinite article is used [1]. For instance,
αὐτὸν εἶναι θεόν = he was a god. (Acts 28:6)
The usage of the indefinite article merely conveys that the predicate noun 'god' is qualitative. In Greek, to emphasise that the predicate noun θεός is qualitative, the definite article is dropped and it is placed before the verb. θεὸς ἦν ὁ λόγος . Acts 28:6 gives no emphasis on the predicate noun θεός as qualitative. This is why ''god'' with the indefinite article isn't an accurate English translation of John 1:1c in Greek.
No article (either definite or indefinite) should be used when translating the anarthrous pre-verbal predicate Greek noun into English. The noun by itself in English is enough to emphasise that the noun is being used qualitatively.
The Word was God. (John 1:1c)
The Word became flesh. (John 1:14a)
The Son of Man is Lord of the sabbath (Mark 2:28).
The translation of theos into English as ''god'' with the indefinite article in John 1:1c fails to accurately translate the Greek into English by failing to emphasise the predicate noun θεος as qualitative in English. It made use of the indefinite pronoun "a" to merely describe what the Word is.
David Bentley Hart accurately translated θεός this way in John 1:1c:
The Word was god.
Alternatively, to emphasise even more that the predicate noun ''god'' is qualitative, we could add an appositive [2]:
The Word was, by nature, god.
Majority of English translation didn't only translate theos into English accurately as ''god'' (without any article) but also translated it precisely as a proper noun ''God''. This is because in the context, the Word was eternal, being the one through whom all things came into being (v. 3). Thus, the anarthrous θεος in John 1:1c should be treated as a proper noun ''God'' [3].
The Word was God.
To give an even more emphasis on the predicate noun ''God'' as qualitative, an appositive can be used:
The Word was, by nature, God.
Alternatively, we can translate the noun θεος (''god'') as ''divine'' in a qualified sense [4]:
The Word was divine.
Conclusion
θεος in John 1:1c should be (i) translated as a proper noun ''God'' because it refers to a deity, and (ii) translated without any articles to emphasise its qualitativeness:
Footnotes
[1] The indefinite article (a, an) is used before a noun that is general or when its identity is not known (http://www.butte.edu/departments/cas/tipsheets/grammar/articles.html).
The noun θεός in John 1:1c is being used in its general sense. In English, to convey that the noun ‘god’ is being used in the general sense (i.e. the qualitative sense), we use the indefinite article ‘a’: ‘a god’. For instance, in Acts 28:6, people thought Paul was ‘a god’ (αὐτὸν εἶναι θεόν, ‘he was a god’….Acts 28:6). Paul was simply being described as a god in the general sense.
[2] An appositive is a noun or a noun phrase that sits next to another noun to rename it or to describe it in another way (https://www.grammar-monster.com/glossary/appositive_apposition.htm).
The use of appositive in the predicate of John 1:1c is very useful since it can help express the meaning of the Greek when it emphasised the predicate noun θεός as qualitative.
[3] When the word θεός refers to the true god, English bibles translate it as a proper noun wherein the first letter in the word ''god'' is capitalised: ''God''.
[4] ''Such an option does not at all impugn the deity of Christ. Rather, it stresses that, although the person of Christ is not the person of the Father, their essence is identical. Possible translations are as follows: “What God was, the Word was” (NEB), or “the Word was divine” (a modified Moffatt). In this second translation, “divine” is acceptable only if it is a term that can be applied only to true deity. However, in modern English, we use it with reference to angels, theologians, even a meal! Thus “divine” could be misleading in an English translation. The idea of a qualitative θεός here is that the Word had all the attributes and qualities that “the God” (of 1:1b) had. In other words, he shared the essence of the Father, though they differed in person. The construction the evangelist chose to express this idea was the most concise way he could have stated that the Word was God and yet was distinct from the Father'' (Emphasis his. Wallace, Daniel. Greek Grammar beyond the Basics: An Exegetical Syntax of the New Testament. Grand Rapids, MI: Zondervan. 1996. 269.).
