There is no manuscript question here. Catholic, Orthodox and Protestant churches read John 1:1 as "the Word was God", meaning that the Word shares fully in what God is while being distinct from the Father ("with God"). The Jehovah's Witnesses' New World Translation reads "a god", arguing from the Greek's missing article. Grammarians who reject "a god" answer in two ways: some (Colwell, Metzger) argue that a predicate noun placed before the verb can be definite without the article; others (Harner) that it marks a quality, "the same nature as God", not "one god among others".
The verse
Text
BSBIn the beginning was the Word, and the Word was with God, and the Word was God.
KJVIn the beginning was the Word, and the Word was with God, and the Word was God.
WEBIn the beginning was the Word, and the Word was with God, and the Word was God.
John 1:1 · “the Word was God / the Word was a god”
καὶ θεὸς ἦν ὁ λόγος (kai theos ēn ho logos); earlier in the verse πρὸς τὸν θεόν (pros ton theon, 'with the God')
In the clause before, "the Word was with God", God has the article (τὸν θεόν). In the last clause θεός has no article and stands before the verb: literally "and God was the Word", where the article on ὁ λόγος marks "the Word" as the subject. Greek has no indefinite article, so an article-less noun can sometimes be rendered "a ...": in Acts 28:6 the islanders say Paul is θεόν, "a god" (KJV). The question is what John's clause requires. The New World Translation (Jehovah's Witnesses) has "the Word was a god", with the study note "Or 'the Word was divine [or, 'a godlike one']'", arguing that the absence of the article "makes the·osʹ qualitative in meaning and describes a characteristic" and denying that the Word is "the same as Almighty God". Grammarians who reject "a god" take two routes. Colwell (1933) argued that a definite predicate noun placed before the verb normally lacks the article, so that here "it is indefinite in this position only when the context demands it", and the context does not; Metzger (1953), following Colwell, called "a god" "a frightful mistranslation" that would make its users "polytheists". Harner (1973) instead agreed that the noun is qualitative but suggested the sense "the Word had the same nature as God". The Catholic NAB (New Testament revised 1986) notes that the "lack of a definite article with 'God' in Greek signifies predication rather than identification", that is, John does not say the Word is the same person as "the God" he is with. Origen, in the third century, already drew a distinction between "God with the article" (the Father) and "God (without article)", even contrasting "God absolutely and a god", while saying that the Word "by being with God is at all times God". The Nicene Creed (325) expresses the reading held by Catholic, Orthodox and most Protestant churches: "God of God, Light of Light, very God of very God". Plainly put: the Greek excludes reading the clause as "the Word is the Father", and the context (the Word as creator in 1:3, Thomas's "my Lord and my God" in 20:28) is what most translators cite against "a god"; but grammar alone does not make "a god" impossible, so the choice is finally theological.12345678910
Confraternity of Christian Doctrine, New American Bible (New Testament revised 1986), John 1 with notes, Libreria Editrice Vaticana (1986) — Note [1]: 'Was God: lack of a definite article with "God" in Greek signifies predication rather than identification.'↩
“New World Translation of the Holy Scriptures (Study Edition), John 1, with study notes”, Watch Tower Bible and Tract Society — Text: 'and the Word was a god.' Study note: 'the Word was a god: Or “the Word was divine [or, “a godlike one”].” ... the absence of the article in this grammatical construction makes the·osʹ qualitative in meaning and describes a characteristic of “the Word.”' and 'there are good reasons for saying that John did not mean that “the Word” was the same as Almighty God.' (Study notes read at https://wol.jw.org/en/wol/b/r1/lp-e/nwtsty/43/1.)
“John 1:1 and the New World Translation (CRI Statement DJ520)”, Christian Research Institute — A compilation of quotations from grammarians (an apologetic source). Philip B. Harner, 'Qualitative Anarthrous Predicate Nouns: Mark 15:39 and John 1:1', JBL 92 (1973) p. 87: 'Perhaps the clause could be translated, ‘the Word had the same nature as God.” This would be one way of representing John’s thought, which is, as I understand it, that ho logos, no less than ho theos, had the nature of theos.' (Harner's article itself was not read.) CRI's entry for Colwell is not used here: its wording does not match Colwell's words as Metzger quotes them (see the next source).
Bruce M. Metzger, “The Jehovah's Witnesses and Jesus Christ: A Biblical and Theological Appraisal”, Theology Today 10.1 (1953), pp. 65–85, at p. 75 and n. 16 — Scan read on archive.org. 'It must be stated quite frankly that, if the Jehovah's Witnesses take this translation seriously, they are polytheists. ... As a matter of solid fact, however, such a rendering is a frightful mistranslation.' Quoting Colwell: 'A definite predicate nominative has the article when it follows the verb; it does not have the article when it precedes the verb. ... The absence of the article [before θεός] does not make the predicate indefinite or qualitative when it precedes the verb; it is indefinite in this position only when the context demands it. The context makes no such demand in the Gospel of John ...' Note 16 cites E. C. Colwell, 'A Definite Rule for the Use of the Article in the Greek New Testament', JBL 52 (1933), 12–21.
Confraternity of Christian Doctrine, New American Bible (New Testament revised 1986), John 1 with notes, Libreria Editrice Vaticana (1986) — Note [1]: 'Was God: lack of a definite article with "God" in Greek signifies predication rather than identification.'
Origen, Commentary on John, Book II, 2 (ANF vol. 9) — 'He uses the article, when the name of God refers to the uncreated cause of all things, and omits it when the Logos is named God.' 'the Word of God, who was in the beginning, and who by being with God is at all times God, not possessing that of Himself, but by His being with the Father'. And, summing up: 'There was God with the article and God without the article, then there were gods in two orders, at the summit of the higher order of whom is God the Word ... And, again, there was the Logos with the article and the Logos without the article, corresponding to God absolutely and a god'.
“New American Bible: the 1986 revision of the New Testament”, New Catholic Encyclopedia (via Encyclopedia.com) — 'The revised NT was approved both for publication and for liturgical use in 1986 and was issued early in 1987'; the Psalms revision was finished in 1991. (Dates the NAB notes cited here.)
Creed of the Council of Nicaea (325) (NPNF 2nd ser. vol. 14) — 'the Son of God, the only-begotten of his Father, of the substance of the Father, God of God, Light of Light, very God of very God, begotten (γεννηθέντα), not made, being of one substance (ὁμοούσιον, consubstantialem) with the Father.'
6The islanders were expecting him to swell up or suddenly drop dead. But after waiting a long time and seeing nothing unusual happen to him, they changed their minds and said he was a god.