John Chrysostom

This is not an interrogation, but an expression of doubt and sorrow. How hath such a course been cut short? who hath been able to do this? ye who were superior to all and in the rank of teachers, have not even continued in the position of disciples. What has happened? who could do this? these are rather the words of one who is exclaiming and lamenting, as he said before, "Who did bewitch you?" (Gal. iii: 1.)

Homily on Galatians 5 ↗

Theophylact of Ohrid

This is not a question, but a lament. He says: you had attained perfection, but what happened? Who gained such power as to hinder you from obeying not the evangelical truth, but the abolished law?

Commentary on Galatians ↗

Oecumenius

"who hindered you?" The "who" is not of one asking, but of one sorrowful, as if he had said: You were reaching perfection, what has happened? Who has had such power as to prevent you from obeying the truth of the Gospel?

Commentary on Galatians ↗

Jerome

You were running well. Who hindered you from obeying the truth? The Latin translator, in his interpretation, put 'not obeying the truth', which is written in Greek as 'τῇ ἀληθείᾳ μὴ πείθεσθαι'. He interpreted it in the previous place as 'not believing the truth', which we noted in its proper place because it is not found in ancient manuscripts, although the Greek copies have been confused by this error. …

Commentary on Galatians ↗

Thomas Aquinas

Then when he says, "You did run well. Who hath hindered you that you should not obey the truth?" he deals with the obstacle to standing. First, he mentions the obstacle; Secondly, he teaches its removal (v. 8). The obstacle to their standing fast was great and harmful, for the harmfulness of anything is reckoned according to the greater good it hinders. Therefore, when someone is kept from many spiritual goods, it is an indication that he is faced with a great obstacle. …

Commentary on Galatians ↗