John Chrysostom

"Neither, because they are the seed of Abraham, are they all children." Now when you come to know of what kind the seed of Abraham is, you will see that the promise is given to his seed, and know that the word hath not fallen to the ground. Of what kind, pray, is the seed then? It is no saying of mine, he means, but the Old Testament itself explains itself by saying as follows, "In Isaac shall thy seed be called." What is, "In Isaac?" Explain.

Homily on Romans 16 ↗

Theophylact of Ohrid

God, he says, fulfilled what was promised. He said: "To you I will give it, and to your offspring" (Gen. 13:15). So then, let us see what this offspring is. Not all who descended from Abraham are simply his offspring, and not all who descended from Israel are Israelites, but those born after the example of Isaac and distinguished by the virtue of Israel, through which he saw God. For he did not say: who are from Jacob, but: "from Israel," mentioning the more honorable name. …

Commentary on Romans ↗

Thomas Aquinas

Then he clarifies his statement, at but in Isaac. First, in regard to Abraham; second, in regard to Jacob, at and not only she. In regard to the first he does three things: first, he cites a text from Scripture, saying, but in Isaac will your seed be called. This the Lord said to Abraham, as it says in Genesis 21, when describing the expulsion of Ishmael. …

Commentary on Romans ↗