...what of ἐὰν μὴ. This is where the KJV fails.
Lexical and Greek Support for ἐὰν μὴ Having the Sense of ἀλλά (“But”) in Galatians 2:16:
Galatians 2:16 in Beza’s 1598 Textus Receptus reads:
οὐ δικαιοῦται ἄνθρωπος ἐξ ἔργων νόμου ἐὰν μὴ διὰ πίστεως Ἰησοῦ Χριστοῦ
The KJV renders the construction:
a man is not justified by the works of the law, but by the faith of Jesus Christ
The following sources specifically support understanding ἐὰν μὴ in Galatians 2:16 as having an adversative sense corresponding to ἀλλά, “but,” or “but only.”
1. Johann Caspar Suicer, 1651, p. 187
Citation: Johann Caspar Suicer [Johann Caspar Schweizer], Ratio syntaxis apud Graecos, selectioribus ex novo maxime foedere et Patrum monumentis illustrata exemplis (Tiguri [Zürich]: Johann Jacob Bodmer, 1651), p. 187.
Under the heading V. Enallage Conjunctionis, Suicer explains that one conjunction may be placed in the position of another:
una Conjunctio pro alia ponitur
He then gives εἰ μή and states:
Pro ἀλλὰ, sed.
Meaning:
In place of ἀλλά, “but.”
Immediately afterward he directs the reader to:
Vide quoque ... Gal. II. 16.
Meaning:
See also ... Galatians 2:16.
Suicer therefore specifically points to Galatians 2:16 in a discussion where εἰ μή is said to stand pro ἀλλά, sed, “in place of ἀλλά, but.” Galatians 2:16 itself has the closely related form ἐὰν μὴ.
Suicer's evidence is especially useful because he is discussing Greek syntax, not merely supplying a dictionary gloss.
2. William Greenfield, revised by Thomas Sheldon Green, 1849, p. 50
Citation: William Greenfield, A Greek-English Lexicon to the New Testament, with additions and alterations, revised by Thomas Sheldon Green (London: Samuel Bagster and Sons, 1849), p. 50.
Under ἐάν, the lexicon states:
Ἐὰν μή, except, unless; also equivalent to ἀλλά, Gal. 2.16.
This is direct lexical testimony. Greenfield/Green explicitly identifies Galatians 2:16 as a passage in which ἐὰν μή is equivalent to ἀλλά, “but.”
The correct printed page is p. 50, not p. 51.
3. Joseph Henry Thayer, 1889, p. 171
Citation: Joseph Henry Thayer, A Greek-English Lexicon of the New Testament: Being Grimm’s Wilke’s Clavis Novi Testamenti (New York: American Book Company, 1889), p. 171.
In his discussion of εἰ μή, Thayer explains that following a negative the expression can have the force of the Latin:
sed tantum
meaning:
but only
Thayer then explicitly states:
ἐὰν μή is so used in Gal. ii. 16.
Thayer therefore recognizes Galatians 2:16 itself as an example of this usage. This is particularly important when Thayer is being appealed to as an authority, since Thayer's own discussion allows the sense “but only” in this verse.
4. Panagiotis N. Trembelas, 1886–1977
Panagiotis N. Trembelas (1886–1977) was a Greek theologian and professor at the University of Athens. He was born in Stemnitsa, Greece, lived and carried out his academic career in Greece, and Greek was his native language.
In an article reproducing Trembelas’s interpretation of Galatians 2:16, the biblical Greek first reads:
ἐὰν μὴ διὰ πίστεως Ἰησοῦ Χριστοῦ
This is the same ἐὰν μὴ construction found in Beza’s 1598 Textus Receptus.
The article then gives Trembelas’s interpretive rendering in modern Greek:
ἀλλὰ μόνο με την πίστη στον Ιησού Χριστό
Meaning:
but only by faith in Jesus Christ
The source therefore places ἐὰν μὴ in the biblical text and ἀλλὰ μόνο, “but only,” in Trembelas’s modern Greek interpretive rendering of the same sentence.
Thus, a native Greek-speaking scholar who was born, lived, studied, and taught in Greece understood the construction naturally enough to express its sense in modern Greek with ἀλλὰ μόνο, “but only.”
Summary
Suicer, 1651, p. 187: explains the substitution of conjunctions, gives εἰ μή ... pro ἀλλά, sed, “in place of ἀλλά, but,” and specifically directs the reader to Galatians 2:16.
Greenfield/Green, 1849, p. 50: explicitly states that ἐὰν μή is “also equivalent to ἀλλά, Gal. 2.16.”
Thayer, 1889, p. 171: explains the relevant construction with the sense sed tantum, “but only,” and explicitly says that ἐὰν μή is so used in Galatians 2:16.
Trembelas, 1886–1977: a native Greek-speaking theologian and University of Athens professor, takes the ἐὰν μὴ of Galatians 2:16 and expresses its thought in modern Greek as ἀλλὰ μόνο, “but only.”
Therefore, saying that ἐὰν μὴ has the sense or force of ἀλλά, “but,” in Galatians 2:16 has substantial grammatical and lexical support. This does not mean that ἀλλά is the ordinary lexical definition of ἐὰν μὴ in every context. It means that in this particular construction and context, recognized authorities have explicitly understood ἐὰν μὴ adversatively, with the sense “but” or “but only.”
Side Note:
Here is a friendly suggestion, my friend. Next time, instead of assuming the KJV has an error, try asking ChatGPT if there is any lexical support in history for the rendering and you will be amazed. Ask if people who lived in Greece, and whose native tongue is Greek also understood it with wording that is supported by the KJV (Note: You may have to connect the modern wording with the older sense of words in the Middle English dictionary in some cases like "faith" meaning "faithfulness."). In other words, once you employ this method, all these so-called errors in the KJV you see will disappear. Granted, some so called errors have to be resolved by looking at the context, though, too.
....