Theophylact of Ohrid

So since they were afraid that by departing from the law they would lose their kinship with the patriarch (for they took great pride in it), he, on the contrary, shows that faith most of all makes sons of Abraham of those who possess it.

Commentary on Galatians ↗

Oecumenius

Since they were afraid to abandon the law, for fear that they fall away from the kinship of Abraham, he turns the argument the other way. The "then," [ἄρα] is "so therefore" [τοιγαροῦν]. "that those who are of faith." Therefore these were especially opposed to the law, scorning its antiquity. It must then be shown that faith was established before the law, and that above all by it one is pleasing to God from above.

Commentary on Galatians ↗

Jerome

So you know, those who are of faith, these are the children of Abraham. He discusses more fully in the letter to the Romans that faith was reckoned to Abraham as righteousness, not in circumcision but in uncircumcision (Rom. 4). And carefully observing, he teaches that those who believed with this mindset are the children of Abraham, just as Abraham believed when he was uncircumcised, who rejoiced to see the day of the Lord, and saw it and was glad (John 8). …

Commentary on Galatians ↗

Thomas Aquinas

From this authority he draws the minor proposition, saying "Know ye therefore, that they who are of faith, the same are the children of Abraham." As if to say: Someone is called the son of another because he imitates his works; therefore, "if you be the children of Abraham, do the works of Abraham" (Jn 8:39). But Abraham did not seek to be justified through circumcision but through faith. Therefore the sons of Abraham are they who seek to be justified by faith. …

Commentary on Galatians ↗