ἐὰν μὴ is a conditional particle meaning “except,” “unless,” “if not.”
In Galatians 2:16 it functions as a strong adversative exception: οὐ δικαιοῦται ἄνθρωπος ἐξ ἔργων νόμου ἐὰν μὴ διὰ πίστεως Ἰησοῦ Χριστοῦ
“a person is not justified by works of the Law except through the faithfulness of Christ.”
The construction is syntactically simple but theologically loaded:
Paul is not saying justification comes by two possible routes (works vs. faith), but that works never justify, and the only exception to universal non‑justification is διὰ πίστεως Ἰησοῦ Χριστοῦ.
The force of ἐὰν μὴ therefore heightens the pistis Christou debate: if the genitive is subjective, Paul’s statement becomes “no one is justified by works, except through Christ’s faithfulness,” which centers justification on Christ’s covenant fidelity; if objective, it becomes “no one is justified by works, except through faith in Christ,” which centers justification on human believing.
Either way, ἐὰν μὴ marks the exclusive gateway of justification, and its semantic weight supports the reading that Paul is contrasting human works with Christ‑mediated justification, not human works with human faith.
The phrase literally means "if not". Except works and unless but they are translations.
The text reads, "But knowing that a person is not justified by the works of the law, if not by faith of Jesus Christ."
In other words, works do not make anyone righteous if not through the faith of Christ.
Out of the 17 occurrences of this phrase it is only translated one time in the KJV as but. And that is here in Galatians. In every instance you can use the literally meaning of the word, "if Not" and retain the premise of the passage as translated in the KJV. But not in Gal2:16. By translating ἐὰν μὴ to but we lose the meaning of the phrase and it changes the understanding of the text for most. The words are not interchangeable in this instance. "But" does work a lot of times but not in Galatians 2:16.
Here are the other times the phrase is used.
John 7:51 Doth our law judge any man,
before (if not) it hear him, and know what he doeth?
Mark 7:3 For the Pharisees, and all the Jews,
except (if not) they wash their hands oft, eat not, holding the tradition of the elders.
Mark 7:4 And when they come from the market,
except (if not) they wash, they eat not. And many other things there be, which they have received to hold, as the washing of cups, and pots, brasen vessels, and of tables.
John 3:2 The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest,
except (if not) God be with him.
John 3:27 John answered and said, A man can receive nothing, except (if not) it be given him from heaven.
John 20:25 The other disciples therefore said unto him, We have seen the Lord. But he said unto them,
Except (if not) I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe.
Acts 8:31 And he said, How can I,
except (if not) some man should guide me? And he desired Philip that he would come up and sit with him.
Acts 15:1 And certain men which came down from Judaea taught the brethren, and said,
Except (if not) ye be circumcised after the manner of Moses, ye cannot be saved.
Acts 27:31 Paul said to the centurion and to the soldiers,
Except (if not) these abide in the ship, ye cannot be saved.
Rom 10:15 And how shall they preach,
except (if not) they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things!
1Cor 14:6 Now, brethren, if I come unto you speaking with tongues, what shall I profit you,
except (if not) I shall speak to you either by revelation, or by knowledge, or by prophesying, or by doctrine?
1Cor 14:9 So likewise ye,
except (if not) ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
1Cor 15:36 Thou fool, that which thou sowest is not quickened,
except (if not) it die:
2Thess 2:3 Let no man deceive you by any means: for that day shall not come,
except (if not) there come a falling away first, and that man of sin be revealed, the son of perdition;
2Tim 2:5 And if a man also strive for masteries, yet is he not crowned,
except (if not) he strive lawfully.
Rom 11:23 And they also,
if they abide
not still in unbelief, shall be graffed in: for God is able to graff them in again.