Paul is astonished. People who had seen Christ crucified set before them so plainly were now turning away from that truth. His question asks how such a thing could happen.

Barnes says they were foolish "for having yielded to the influence of the false teachers." Gill says Paul means "their present stupidity about the article of justification," not a national character. Barnes, by contrast, notes that ancient writers also called the Galatians "a foolish people." "Who hath bewitched you?" pictures them acting as if under a spell. Barnes explains the word as "to mislead by pretences, as if by magic arts; to fascinate," and Gill notes it is a figure of speech. Paul is not accusing them of witchcraft. He is asking who cast such a mist over their eyes.

No reply is recorded. Paul has already pointed to who was troubling them: "there be some that trouble you, and would pervert the gospel of Christ" (Galatians 1:7). Instead of waiting for an answer, he presses further: "This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith?" (Galatians 3:2). He asks a similar question later: "Ye did run well; who did hinder you that ye should not obey the truth?" (Galatians 5:7).

"Foolish" is anoetos, thoughtless, not using one's understanding. "Bewitched" is baskaino, to give the evil eye, to fascinate. "Evidently set forth" is prographo, to write beforehand or announce publicly. Barnes says Christ had been preached among them "as if the doctrine was set forth in a public written tablet," and "as plain as if there had been a representation of him by a picture."

The Textus Receptus, the Greek text behind the KJV, includes "that ye should not obey the truth" and "among you." Many modern texts, which the ESV follows, leave them out. Barnes himself notes the first phrase "is lacking in many manuscripts." The same idea appears in Galatians 5:7, so the meaning is not affected.

Paul's question puts the cross at the center. The Galatians had not received a list of rules from him. They had received Christ crucified. Gill says Christ "was the sum and substance of his ministry." To turn from that to the works of the law was to act as if the cross were not enough. Paul has just said that if righteousness came by the law, "then Christ is dead in vain" (Galatians 2:21).

The sharp tone comes from love. The same letter calls them "My little children, of whom I travail in birth again until Christ be formed in you" (Galatians 4:19). Paul's rebuke aims to bring them back to the One they first saw so clearly.