Theophylact of Ohrid

If the law, he says, grants blessing and makes heirs of life and righteousness, then the promise given to Abraham is consequently abolished and destroyed. But this would make no sense: the law appeared later, so how can it annul a covenant that existed before it? But do not hasten to press this entire example too strictly. For this reason he also said: "I speak in human terms," that is, I am giving a human example. Therefore it is not surprising if it cannot be fully applied to the divine.

Commentary on Galatians ↗

Oecumenius

"For if the inheritance is of the law." He himself so interpreted it. For if the blessing came through the law and not through Christ, the promise and covenant of God is annulled and terminated. He gave by the promise. He gave the blessing, he says, by the promise of God and not by the law.

Commentary on Galatians ↗

Thomas Aquinas

Then when he says, "For, if the inheritance be of the law, it is no more of promise," he shows how from the foregoing it follows that the Law would nullify the promises, if the Law were necessary for justification or for the blessing to come to the Gentiles. He says therefore: The promise would indeed be disannulled, if the Law were necessary; for "if the inheritance," namely, of Abraham's blessing, "be of the law, it is no more of promise," i.e., of the seed promised to Abraham. …

Commentary on Galatians ↗