Theophylact of Ohrid

The promise of God to Abraham was fulfilled, who said to him: "and in you shall all the families of the earth be blessed" (Gen. 12:3). Abraham was indeed multiplied now, when all the nations through faith became his sons; for as he believed, so they too through faith became his sons.

Commentary on Luke ↗

Bede

As He spoke to our fathers, to Abraham and his seed forever. The seed of Abraham signifies not the carnal, but the spiritual, that is, not only those born of his flesh, but those who follow his footsteps in faith, whether in circumcision or uncircumcision. For he believed when in uncircumcision, and it was counted to him as righteousness. And he received the sign of circumcision as a seal of the same faith, so that he might thus become the father of both peoples through faith, as the Apostle most fully explains to the Romans. …

On the Gospel of Luke ↗

Bonaventure

Then is added the true promise, when it is said: As he spoke to our Fathers, to Abraham and to his seed forever. And this text should be construed thus: As he spoke to the Fathers, and as he spoke to Abraham in particular, and to his seed, where it touches upon the state of nature and of the written law. — Or otherwise, so that it is construed with the preceding: He has received Israel, being mindful of his mercy, and this, I say, to Abraham and to his seed, as he spoke to the Fathers. …

Commentary on Luke, Chapter 1 ↗