John Chrysostom

Here he hints that his absence had been the cause of this, and that the true blessing was for disciples to hold right opinions not only in the presence but also in the absence of their master. But as they had not arrived at this point of perfection, he makes every effort to place them there.

Homily on Galatians 4 ↗

Theophylact of Ohrid

Do you not see, he hints at the fact that they rivaled everyone in perfection when Paul was among them? And he also gives to understand that his absence gave rise to this evil. It would be excellent, he therefore says, if not only in the presence of the teacher, but also in his absence, the disciples thought what was proper.

Commentary on Galatians ↗

Oecumenius

He wishes to show that those who now are overturning them, and persuading them to observe to the law, and by this preparing them to become zealous imitators of it, had long ago envied them in the knowledge of the Gospel, when Paul was present and was teaching them. "It is good, however," he says, "to strive enthusiastically in what is good always, and not only when I am present with you." And he shows that they have been deceived during his absence.

Commentary on Galatians ↗

Thomas Aquinas

But he rejects this when he says, "But be zealous for that which is good in a good thing always." As if to say: You ought not to be zealous for them in their teaching; but be zealous for a good teacher, i.e., for me and those like me: "And who is he that can hurt you if you be zealous of the good?" (1 Pet. 3:13). But because there can be evil in a good teacher, he adds, "be zealous of the good" teacher, yet say "in a good thing," i.e., in that which is good: "Follow after charity and be zealous for spiritual goods" (1 Cor 14:1). …

Commentary on Galatians ↗